Know before you go:
For a meaningful memento, select an item you can't easily get athome, something representative of the place you're visiting. Read up on your destination and its regional specialties. And connect with people who've been there: You'll find great info on Lonely Planet's message board, The Thorn Tree (thorntree.lonelyplanet.com).
Stick to things you already like. Are you an art person? Look for pieces by local painters or sculptors. A foodie? Try an indigenous spice you can cook with when you get home. That way, you're much more likely to appreciate your purchase later on.
Prioritize your purchase:
Make time to shop during your trip. When you find something you like, buy it right then--you might think you'll go back for it later, but if you don't get the chance, your only souvenir will be regret.