First of all; make a list. Then cross off half of what you wrote down. Everything you bring should fit in one bag, and the one bag shouldn’t be bursting at the seams. If you have two of something, bring one. If you might need it, you don’t. If you think you really need it, you probably still don’t. If you’ll die without it, okay – bring it.
Pick a color scheme for your clothes; if you base all your clothes around the same color then everything will match.
Roll your clothes; this works best for backpacking and other times when your clothes have to fit in something that’s not a geometric shape.
Follow the rule of 3. The rule is: bring 3 shirts. They can vary depending on the climate of your destination(s).
When it comes to shoes, try and pick one pair that matches everything you own. Shoes are probably the most awkward and bulky things you’ll have to carry, so the fewer the better. A shoe bag will keep them from getting too friendly with your clothes.
Electronics are essential to most travelers. Make sure to check the voltage and plug-style of where you’re going, and buy the most compact and efficient converter available. For trekking and backpacking, a few solar options are available, like the Novothink solar powered iPhone and iPod charger.
If you’re going to be without clean water, bring a SteriPEN. This tiny gadget can sterilize a half-gallon of water in less than a minute.
When you decide which toiletries are absolute necessities, bring as little of them as possible. Buy smaller plastic bottles for shampoo, conditioner, or a space-saving shampoo-conditioner-hybrid. Bring only a half-tube of toothpaste, and forget anything that’s purely cosmetic. If you’re really into saving space, look for shampoo bars and toothpaste powder. Solids are generally more transportable than liquids. When you’re traveling, it’s important not to focus on looking your best or being squeaky clean on a daily basis. Leave your makeup and vanity at home too.
Finally, don’t freak out if you forgot something. Most countries sell everything and anything you need, and waiting until you arrive to buy some things can actually be the best space-saving technique.