I would HIGHLY recommend this taillight:
http://www.performancebike.com/shop/...tegory_ID=4322

It is so bright you wouldn't believe it and the batteries (takes AAAs) last quite a while.

It's a pretty normal clip-on kind, easy to take off, you could probably clip it on a belt or something fat like that (not sure it would stay on a pocket). It's on sale for $13.