Learn about the history.. You need to get yourself a guide to take you around, otherwise you'll miss the history and stories of many things you see. When we went we found a free guide who took us around as long as we ate lunch at their restaurant (not sure if it was really required, but we did anyways).
If you can't find a guide or don't have the time.. at least see one of the temple's and visit the back alleys. about 10 years ago
I have been to SF a phew months a go, was fun to walk around the neiborghood, buy some souvenirs, see temples and one of the most curious things we have seen was the chinese community lottering in Portsmouth Square. Chinese love lottering!
Anyway you can find more information at http://www.sanfranciscochinatown.com/ and http://www.onlyinsanfrancisco.com about 10 years ago