WiFi is one of the biggest users of battery life. Next, GPS also uses battery life.
Also, turn the screen down as low as you can, while still being able to view the screen.
I'm amazed at the battery life for the Droid. Considering the size of the screen, I'd expect it to be lousey, but I used it from about 8am until returning home today, around 7pm, and I used it a LOT (had a minor auto accident, was rear ended by the guy behind me, who was rear ended... ) and still had about 40% battery life left!
The Droid is a monster when it comes to battery life! (I came from a G1 and myTouch, both of which would be on their last legs when I returned home late at night...)