Ive tried a couple very good custom roms and would love to keep them on my phone but they drain my battery so bad I always end up reverting back to my stock rom. Why do the roms use more power and is there anything I could do to improve the battery life with a custom rom?
Which custom roms are you using? I've had much better battery life on custom mods than I did stock. Getting a good kernal will really help with battery life as well. What kernal you want will depend on whether you have a sense based rom or an AOSP rom. For sense, I'd suggest Redemptive Revolution ROM with the included kernal (lou's #9). For AOSP, I'd go with Cyanogenmod 6.1 or 7.
Also, if you're running an app killer, you should probably uninstall that as they aren't needed and use up battery.
Also, try looking around in the Rooting, ROM's, and Hacks section at the top of the forum. This has been beat to death up there and a lot of people that know what they are talking about have weighed in...