I've noticed that the battery life goes to *insert your favorite explicative here* at random times (this time it's been for almost a whole week of it like that) because the CPU is pegged at 100% running filesys.exe. Although, I don't run the T-mobile ROM... I use the Touch-IT ROM.
EDIT: I forgot to mention.
1. Bring the screen brightness down.
2. Get BatteryStatus and downclock the CPU when you don't need the extra CPU power (most of the "phone" time.)
3. Don't leave the EDGE/GPRS/WIFI/Bluetooth on.
4. Disable MS Live... wait... that's not a battery thing, but the thing is a memory leak and hog.