i agree with powerkiter about task killer apps. i used to use atk but it just gave me an ocd. of course that was when my x10 used to last half a day. i've owned mine for about a month now and it lasts 24 hours easy with fair use.
many people say that an alternative home launcher (i'm usind adw launcher, it's fantastic) helps battery life so i have one.
i also have juice defender. this app is great cuz it turns 3g on in intervals and when the screen is unlocked