The first thing to try is:
phone settings > location & security > use wireless networks > UNCHECK
If that doesn't seem to improve battery life go ahead and CHECK the above setting but do this religiously:
-turn off GPS when done using it
-turn off WIFI when done using it
-reboot phone after charging it
All of this stuff should not be necessary. On my Droid 1, I leave GPS and WIFI on all the time. People have reported that this is fixed in the leaked 2.2 update for Droid X so after the update you'll be able to leave it all ON.
See here for more info: Droid X battery problems discovered!!!!!!!