From now on, I’m travelin’ light.
How To Pack It
Bottled liquids (and gels) of any kind are leakage prone, all the more so on airplanes: it's truly surprising what can be made to leak under changing air pressure conditions. Full containers fare better than those containing large air spaces; one helpful trick is to squeeze the container a little before screwing on the cap, thus giving the contents more room to expand).
You can improve the seal by placing a piece of plastic film over a bottle opening before screwing on the cap, a useful thing to know if you must use the original container for some reason.
Better, though, is to repackage liquids and gels in superior (and smaller) containers. Not only will this reduce the leakage problem, it will save weight and space. You are, after all, unlikely to need a six-month supply of shampoo on your two-week trip to Greece; for that matter, it is possible to buy shampoo in Greece. Repackaging is also a more environmentally responsible approach than buying products in miniature "travel size" containers. There are a couple of repackaging options worth considering …
For a fairly rigid, more traditional container, consider the lightweight, unbreakable high-density polyethylene (HDPE) bottles made by Nalgene®, especially the narrow-mouth variety (pictured at left). These feature particularly well-designed caps, about as leakproof as you're likely to find (REI and Campmor carry these in a variety of sizes), thus the most effective solution for thin, "runny" liquids. The downside of all such containers, of course, is that they remain the same size as they are emptied, wasting space and entrapping air (which may not be best for the contents).
For more viscous liquids, a preferred solution is the use of leakproof, refillable squeeze tubes. As you might imagine, such a product is not easily come by, but Easy Traveler makes a robust, surprisingly well designed version (shown at right). Part of what makes them leakproof — and their contents more accurately dispensed — are their relatively small exit holes; these would also make them impossible to fill, except that the company supplies a wide variety of couplings to attach them to just about any source container imaginable (and a sort of "hypodermic syringe" for use if those fail). The sturdy tubes are available on their own, but best purchased initially as a kit that aids in their use (most appropriate for the air traveller is their 3-1-1 Totally Compliant Carry-On Kit™, with ten assorted squeeze tubes, a spray container, and an extensive set of container transfer fittings).
What about those cute GoToob® (and similar silicone-based) containers? Despite their undeniable tactile appeal, I'd avoid them: they are impractical, for a number of reasons. They are incompatible with an astonishing number of products. They are not particularly leakproof. Their caps are notoriously brittle, and unlikely to survive being dropped on a tile floor. They are effective dirt magnets. They are air-permeable, so liquids will dry out if kept in them for an extended period. But the most important concern is that those chunky GoToobs are quite heavy, almost four times the weight of the containers that I recommend. If nothing else, then, these bottles serve as an informative indicator: anyone packing them is not seriously striving to travel lightly!
Repackaging can be fussy and time-consuming, to be sure, but if you insist on carrying liquids, it's probably the best way to go. There are alternatives, however: see below.
Finally (and yielding in this rare instance to the "umbrella plus raincoat" philosophy), I strongly recommend placing all containers that hold liquids inside one or more sealed plastic bags as well, just to be sure. Although the traditional Ziploc® bag (especially the heavier versions intended for freezer use) can be used for this purpose, a much more effective (and durable) solution is the aLOKSAK bag (shown at left), which is certified waterproof to depths exceeding 60 meters, for periods of at least two weeks. An aLOKSAK can also serve as an ice pack, a secure food storage container, and (when inflated) a pillow; in fact, leaving some air inside a sealed aLOKSAK is a good way to provide extra cushioning for items packed within. You can even reconstitute or cook food in one, by pouring boiling water into the bag (it should not be immersed in boiling water). Should your personal aesthetic prefer removal of the product's lettering, this can safely be achieved with denatured alochol.
Liquids and Air Travel Security
In late 2006, the U.S. Transportation Security Administration introduced some draconian (and annoying) rules pertaining to the carrying of liquid, gel, and aerosol products aboard commercial aircraft. The 6×9-inch version of the aforementioned aLOKSAK bag (which sells for under USD$3) is an excellent candidate for the "one, quart-size, zip-top, clear plastic bag" that air travellers must currently use in order to carry liquids, gels, and/or aerosols through security inspections.
If you do wish to carry aboard a larger-than-permitted amount of some liquid or gel, remember that you can repackage it into multiple smaller, legal-sized containers (as discussed above), as long as all of them fit into the requisite bag. Understand also that medically necessary liquids, gels, and aerosols (baby formula, insulin, cough syrup, contact lens solution, prescription medications, etc.) are generally exempt from security restrictions, though your inspection will take longer as a consequence. Nonetheless, the original packaging for such things is usually not very satisfactory, and you will benefit from transferring them to more travel-friendly alternatives, as explained above.
I'm not at all convinced that a snow globe makes a good souvenir, but if you feel otherwise, it will have to go in checked baggage.
Alternatives to Liquids
The best solution to all of the above concerns, of course, is simply to eliminate the carrying of liquid (and other restricted) products in your luggage altogether. This is much simpler than you might think: most toiletries, cosmetics, etc. come in solid form, and the use of these avoids the weight, bulk, and environmental cost of excess packaging, in addition to the considerable weight and bulk of the liquids themselves (water is heavy).
Here are some example products/suppliers: