I've used the crest white strips before...they do a decent job but they only get like the front 6 teeth which is pretty annoying. On a different thread in this forum tiger88 had a great recommendation:
Goto the store like wal-mart, buy 2 rubber mouth guards. Take them home, follow the instructions to mold to your teeth. Then you can either buy some of the Rembrandt whitening gel (which comes in plastic syringe things) or you can go on ebay and buy the 22% carbamide solution stuff Synpax was talking about. Put the gel in the mouthguards 20 mins a day 'till your teeth are white to your liking. I've been doing this for a while now and it works fuckin' perfect. I do it like 4x a week maybe once every month or two and they stay white.
Most cost effective method I've seen.