Pack less, and become unattached to possessions.
And then … pack less.
What To Pack It In
Day Travel Bags
Whatever type of bag you choose, consider tucking in a lightweight, highly-compressible day travel bag (daypack). On leisure travels, this gives you the ability to leave the main bag at your hotel, B&B, pension, or station locker, and carry your around-town necessities (rain jacket, sweater, water bottle, guidebooks, etc.) in the daypack.
Many business travellers find such a bag equally useful: it can, for example, provide a convenient place to keep items that you will use during the flight separate from everything else, allowing you to quickly remove them from your main bag before storing the latter in the overhead compartment. When not in use, a daypack of the type I recommend takes up very little space in one's bag (some of the more compressible bags come with separate stuff pouches, though I prefer to simply pack them flat when not in use, thereby reducing hassle and saving weight).Daypacks tend to be a more personal choice than other bags, thus it's not as easy to suggest an "ideal" solution. Some people want an unobtrusive bag with modest carrying capacity, while others prefer to keep their options open with a more generous amount of storage space. Some want lots of compartments and bottle holders, while others crave a single unconstrained space.
Recommended Day Travel Bags
In their attempts to meet the dual goals of compressibility and reduced weight, the bags I suggest are not as rugged as many that you can buy, or festooned with as many "features" (neither are they as expensive), though they are surprisingly strong for their weight. Unlike the main bags I recommend, they are unlikely to last a lifetime, though will provide many years of valuable service for those who travel lightly.
Travellers who — like me — prefer a less structured daypack (making it more lightweight, compressible, multifunctional, and back-hugging) will do well with the various ultralight cylindrical styles, despite what some consider a somewhat unconventional appearance. My favourite is the under-$26 "Ultralight Travel Daypack", from Hammock Bliss. Measuring 20×11×7 inches (51×28×18cm) and weighing a miniscule 6.5 oz. (184g), it provides a capacious — and unobstructed — 1100 cubic inches (18L) of storage in its top-loading main compartment, and also includes a zippered inside pocket for small items, plus two mesh side pockets (visible in the photo at left); the right-side pocket incorporates a top closure, allowing it to double as a storage pouch for the bag (simply turn the pocket inside out with the bag inside). Lastly, and importantly, it features well-designed, comfortable, double-padded shoulder straps that incorporate a carry handle.
REI also makes a couple of very nice packs in this style (albeit more than 50% heavier): their "Flash 18" and "Stuff Travel" models.
An excellent offering along more familiar lines is Kiva Designs' "Rick Steves Civita Day Bag" (shown at right), which comes in several colours and sells for under $20. Obviously designed by someone with plenty of real-world travel experience, it collapses to almost nothing, yet provides over 900 cubic inches (15L) of storage space in three differently-sized compartments, surrounded by a soft microfiber fabric that allows it to be used as a pillow when stuffed with a jacket or sweater. With its lightly padded shoulder straps and two mesh bottle holders, it measures 13×10×7 inches (33×25×18cm), and weighs 8.6 oz. (244g).
Another nice (and more sleek/compact) design, having benefitted from several revisions over the years, is the "Eagle Creek Packable Daypack" (pictured at left), which retails for under $25 and also comes in a choice of colours. Its 210 denier ripstop fabric encloses a modest 700 cubic inches (11L) of volume in a single main compartment; a flat front pocket with a reversible zipper doubles as a storage pocket, in much the same fashion as the Hammock Bliss bag, above. The 6 oz. (170g) bag sports breathable mesh shoulder straps and a key fob, but no internal or water bottle pockets.

Those wanting still greater carrying capacity should consider Barefoot Enterprises' "Wanderlite Packable Daypack" (shown in use at tight), which sells for under $28. Its spartan teardrop-shaped design encloses — primarily in a single large compartment — a generous 1925 cubic inches (32L) of storage space (18×14×8 inches, 46×36×20cm), yet weighs only 8 oz. (227g). The 420 denier nylon packcloth fabric is somewhat stronger and more weatherproof than the Civita's, though is (unfortunately) partnered with lower quality zippers. Its wide shoulder straps are unpadded, though surprisingly comfortable.
Whenever you have occasion to temporarily remove your daypack, and place it on the floor (at a restaurant, say), a good habit to cultivate is passing your leg through one of the shoulder strap loops. Not only will this hamper anyone trying to grab the bag, it will also remind you not to leave it behind when you move on (this being the primary cause of lost packs). Don't do this on an airplane, though: it will impede your exit in an emergency situation.
A screw-lock mountaineering carabiner can also be employed to temporarily secure a daypack.
Finally, a daypack also makes a good emergency bag (as an alternative, or even addition, to a small duffel), useful if your return voyage finds you with considerably more stuff than you started with. Don't forget, though, that mailing items home can often rescue you from this problem.
