Depends on where you live and where you plan on looking. College towns are awesome for dumpster diving, and so are cities. A sure way that should work no matter where you are is to contact the town halls in your area and find out when they schedule large garbage collection days. Many have a monthly routine, and a schedule will help you coordinate your dives. Some finds are so sweet its like trick or treating for adults!
I heard a site called
www.freecycle.org is pretty good too and
www.craigslist.org for your area might have stuff.