The day on which one starts out is not the time to start one's preparations.
How To Pack It
Useful Packing Tips
If you've done a good job of selecting what to pack, the task of actually packing it should be a fairly straightforward one (most difficulties are the result of the former, not the latter). Aside from suggesting that you pack larger items before smaller ones, there's not much I can say about the packing process itself: it's very much a function of the size and design of the bag, plus the specific items being carried on any particular trip. Some folks have good spatial visualization skills, and pack very well; others have to work a bit harder at it. Fortunately, one benefit of developing your own personal packing list is that you don't need to keep (re)solving this problem every time you pack!
All that said, here are a few ideas that you may find helpful …
I've never quite understood what happened to "organizer pouches": simple, flat, rectangular, nylon bags with zipper closures, such as shown at right. Sometimes multi-compartmental, incorporating see-through mesh panels, and/or available in a variety of colours, they are terrific for avoiding what could otherwise be a cluttered mess in your bag, and make life easier for security inspection folks as well (always a good idea). They also permit you to organize your belongings into categories, and make smaller items less likely to be lost or forgotten. One of my pouches, for example, contains all of my laundry-related items (i.e., sink stopper, detergent, spot remover, viscose towel, travel clothesline, inflatable hangers), making it trivial to include or exclude this functionality on any given trip.
But ever since Outdoor Research abandoned production of their terrific versions of these, the field has thinned, and the pouches have become more fancy, more fussy, and more costly (the seriously budget-constrained could use heavy duty Ziploc® bags for this purpose, but I have always preferred the convenience and durability of colour-coded nylon pouches).
Eagle Creek's line of Pack-It® Sacs is probably the most reliable source of organizer pouches these days, although they incorporate snaphooks & tabs that should likely be cut off (unless you have a specific use that warrants their added bulk and weight). If you're willing to spend the time searching, you can certainly find similar products, but the quality varies widely, and I'm not comfortable recommending any in particular; should you come across a reliable, quality supplier of organizer pouches, though, I'd love to hear about it!
An appropriately-sized organizer pouch also makes an excellent core object for the bundle wrapping method of packing clothes. Eagle Creek's large-sized pouch is well suited to this purpose, as is the multi-compartment, equally large-sized "Deluxe Pocket" (made by On Sight Equipment, and pictured at left). Both of these pouches are about 10 × 14 inches (26 × 36cm) in size, and come with snaphooks & tabs, which might better be removed for use as a core object.
Avoid overly-large-sized pouches (as they encourage you to fill them), and if you use more than one, ensure that they are differently coloured (facilitating easy differentiation).
Your selection of cubes will depend on how you choose to organize things; fortunately, they come in a wide variety of sizes. I typically carry three.
The first, used mostly for toiletries, is a Pack-It Quarter Cube (from Eagle Creek, and pictured at right). A mere 7.5 × 4.5 × 2.5 inches (19 × 11 × 6cm) in size, it is dwarfed by the giant "Dopp kits" (and worse) that many people seem to consider necessary for this purpose. But mine quite handily holds all of my toiletry kit, plus vitamins, medicines, etc. with considerable room to spare. And its 300 denier poly micro-weave construction adds only 2 oz. (57g) of weight for its 75 cu. in. (1.2L) of storage space.
In actuality, I have reduced even that weight, by spending a bit (about 50%) more for one of Eagle Creek's newer "Specter" line of cubes, made from extremely thin, translucent white, ripstop Silnylon. Their Specter Quarter Cube (seen at left) is dimensionally equivalent to the regular model, but weighs a whisper-light 0.6 oz. (16g).
I also employ a Pack-It® Tube Cube (also from Eagle Creek) to carry miscellaneous items, things that don't logically group in other ways (such as my flashlight, Jakstrap, whistle, compass, alarm clock, sunglasses, eyeglass retainer, water cup, garbage bags, etc.). This bag is 13 × 4 × 3 inches (33 × 10 × 8cm) in size, weighs 3 oz. (90g), and fits neatly along one side of my carry-on. Sort of my "junk drawer".
Finally, I carry a small see-through packing cube to hold miscellaneous electrical gear: computer power supply & cord, cube tap, mouse, network cables, flash memory card adapter, earphones, remote control for presentations, etc. The Cable Management™ 10, made by Think Tank Photo (and pictured at right), is a good choice for this usage; it measures 7.5 × 3.75 × 2.25 inches (19 × 9.5 × 6cm), and carries a lifetime "no rhetoric" warranty.
As with organizer pouches, eschew large packing cubes (that's what luggage is for) or multiples of the same colour.
And don't just buy a bunch of these and then try to figure out what to do with them: that's pretty much putting the cart before the horse. Decide how you will use them (if at all), then decide what size/shape you'll need, and then buy individual cubes to meet those specific needs..
The use of packing cubes (and organizer pouches) also increases the likelihood that your belongings will remain tidy and well organized should you be singled out for a bag search.
Coping with Shoes
If you're packing shoes (in addition to those you're wearing), consider the use of shoe bags: lightweight, inexpensive, drawstring bags designed to keep any residue of Parisian streets away from the rest of your belongings. You can buy commercial ones, at various price points, or opt for the budget solution: a pair of the plastic bags in which (many) newspapers are delivered: they're sturdy, long, not too wide, and disposable when their days are done. Hoagie/sub/hero sandwich bags can often be used in this fashion, and, in a real emergency, the disposable shower caps provided by many hotels can be called into service as well.
Yet another alternative is the use of hospital shoe covers (pictured below), which are around 10¢/pair when purchased in bulk (a box of 100 will last for many miles of travel). Although they do not completely cover the shoe, these are extremely light (a pair weighs 0.4 oz / 10g), highly compressible, and have the added advantage of multiple uses: they can also serve as temporary footwear for toilet/bath/shower visits when at a hostel or B&B, and will keep your socks clean when traversing security checkpoints that require you to remove your shoes. Remember to turn them inside-out when using them to pack dirty shoes.
Shoes are perhaps the most difficult items to pack, given their typical weight, bulk, and awkward shape. Minimize the number you take, and try to wear (rather than pack) the heaviest pair when reasonable. It is usually best to pack a pair of shoes tightly together, soles out, with the heels at opposite ends. Also, don't neglect the spaces inside your packed shoes: they're great for fragile items, or anything else that will fit, for that matter (small, loose items like jewellery are best grouped in small plastic or fabric bags to keep them from rattling and/or wandering off).
Another useful shoe-packing idea that will appeal to some (especially those wanting to carry athletic shoes) is to attach them to your bag rather than pack them inside. Simply tie the laces together, passing them through the handle (or other convenient attachment point) of your bag, allowing the shoes to dangle from same. Not only does this save significant space in your bag, it also relegates odours, dirt, etc. to the outside of the bag rather than having it share space with your clothing (thus eliminating the need for shoe bags). This trick can also be useful in emergencies, should your going-home bag turn out to have less room than your going-away bag.
Sorting Out Souvenirs & Gifts
In my view, the very best souvenirs are friendships, photographs, and fond memories: those who travel lightly are not typically looking to add more stuff into their lives. If you feel the need for something more concrete, though, or can't pass up a flea-market bargain, you might want to put some thought into what to get, and how to get it home (or haul around with you).
Locally-produced items tend to make the best mementos: "Made in China" is fine if you're visiting that country, but it's otherwise suggestive of a lack of imagination. If you're in Belgium, think lace or chocolate; when in Ireland (or Portugal), think hand-knit sweaters; and so on. Functional souvenirs are usually preferable to purely decorative ones: clothing is often a good choice, and can even replace something you've brought. Readers will appreciate local books on topics of interest, and music-lovers should consider recordings of local musicians they've enjoyed.
I always leave some space in my bag for gifts, souvenirs, and other items that I might want to bring home. Should that prove insufficient, I have the option of using my daypack or lightweight duffel to carry additional stuff. On an extended trip, I will mail items home.
… niceties of Indian, Persian, and Turkish workmanship that they will show you and that you will want to buy, as much to hold on to the memory of the holy voyage, as to share it with your friends. And what you have bought will give you irritation and hardship to bring home.
Buddy Packing & Such
If you have a truly compatible travelling partner, consider packing half of each person's things in the other's bag. That way, in case of a bag being lost (much less likely for the carry-on traveller), each of you will still have half of his/her stuff. This method does, however, have the downside of providing something in the way of a disincentive to pack minimally!
Whether or not you do buddy packing, it should be obvious that when two or more travel together, many of the packing list items (alarm clock, guidebooks, repair supplies, etc.) need not be replicated for each person. And everyone will have access to a more varied wardrobe … even the dissimilarly sized or gendered: a man's shirt can easily be worn by a woman, even (perhaps augmented with a belt) as a dress by a teenager; an older sibling's top makes a younger sibling's dress; etc.
For the sufficiently adventurous, even more creative garment-sharing techniques are possible, as suggested by the video below, demonstrating five non-intuitive ways for a woman to wear a man's shirt. Caution: This video includes a tiny amount of (extremely modest) partial nudity, so don't view it if that might offend you.
Finally, two people travelling together are well advised to carry different credit cards: the same types of cards with differing account numbers (as, for example, allowed by American Express), or different types altogether (say, MasterCard and Visa). This way, should one person's card(s) be lost or stolen, cancelling those card(s) — which, of course, should be done immediately — does not prevent the other person from making credit purchases.
Miscellaneous Packing Thoughts
A place for everything and everything in its place.
Whenever I check into a hotel or the like, one of my first actions is to completely clear at least one horizontal surface (desk, table, dresser top, etc.), and place all the removed clutter out of sight, often in a drawer. I then use this cleared area for any non-clothing items of my own that must be unpacked. That way, when I prepare to depart, I am unlikely to leave something behind because it "disappeared" among a bunch of sight-obscuring oddments.
When you're packing (as opposed to wearing) inclement-weather gear, and there is the slightest chance of such weather occurring, ensure that such items go in an easily-accessible part of your bag: you don't want to be caught in a downpour, unwrapping a clothing bundle to get at your poncho or rain hat.