My house is in a spotty 3G area of town and I get hugely better battery life by keeping wifi on when the phone sleeps so it is not trying to hit the cell tower for email updates etc. It can add two hours a day on the standard battery.
You go to
-->Settings
----->Wireless&networks
--------->Wi-fi settings
--------------> (hit menu)Advanced
------------------>wi-fi sleep policy
----------------------->never (phew)
|