I second what NoSpin says about the data connection. My office is like the black hole of cell phone reception. I turn off 3g and turn on wifi while I'm at work, and my battery lasts much longer by doing this.
The phone will already change its clockspeed based on usage and sleep state. But if you can live without instant mail and other notifications, try Juice Defender. It will **** down your data connection when the phone is asleep and periodically wake up for syncing. Supposedly that saves battery life, at the expense of instant email and other notifications.