I know this has been posted already, but people can't seem to find it so I thought I will put it up after typing it out for another thread. Doing this made a huge difference for me, your mileage may very.
Here is what I did, and why for those who care.
--- EDITED ---
0. Make sure your phone is charged to 100%
1. In ClockworkMod go to advanced, wipe battery stats (or, as of BB 0.6.2 you can go to settings, rom toolkit, troubleshoot, wipe battery stats - AKA PETE rocks and makes things easier)
2. Use your phone till it dies (screen goes blank, ignore the battery percentages and low battery warning, if your stats were messed up you can run on "x" (5%) for awhile, I went about an hour on "x")
3. Charge your phone 100%
4. Your down. Don't run your phone dead more than once as it will not help (my understanding) and it isn't the best for the battery.
--- EDITED ---
The phone uses the stats to learn how much juice your battery can hold to show you percentages. When you wipe before flashing you reset those stats, and depending on how much battery you had left when you wiped and when/how you charge it right after that can pull those stats "smaller". By running the phone to true dead you get more accurate stats and therefore your phone will not tell you your battery is at 15% when it really is at 50%. So it looks like your battery life is better, when really you are just using "more" of battery (aka not charging your 50% full battery because the phone is complaining while thinking you are at 15%).