It shouldn't really be all that bad even if you have Wifi and Data on though.
If you have any kind of task-killer or app-killer program installed, DELETE IT! That will drain your battery like crazy, despite what the program is supposed to do.
Try adjusting your sync settings so you're not constantly refreshing your email etc. as suggested.
I can highly recommend rooting your phone. I consistently get 24 hours or a little more out of each charge with light use. Rooting my phone was the single greatest boost in battery life, and I keep sync/data on all the time, and turn wifi off if I am sure I am not going to use it. The best ROMs I have found for battery life:
1) Virtuous Unity; This one gave me REALLY nice battery life. It has a really good cpu control daemon that really sips your battery juice.
2) CM7; This one gives slightly less battery life, but I still usually get 24 hours or more per charge. I also like this one better for the customization it offers. Really cool.
There's some good tutorials for rooting online, just do a lot of research before you do it though.