Arguably the most impacting variable for battery life on your phone is usage of the radio (WiFi, 3G, 2G, etc). This is assuming a properly functioning phone where a particular bugged application is disallowing the phone to sleep (as the Calendar application in the stock 2.1 ROM does). What a lot of people do not know is that WiFi battery consumption can be substantially effected by your router configuration.
So here it is: Make sure WMM (WiFi Multimedia) is enabled on your wifi router.
Part the WMM spec is based around power saving features on devices such as our phones. All the 3rd party firmware supports it (dd-wrt, Tomato, etc) and many of the more recent routers do out of the box. Chances are you'll find the option to enable it in the QoS section of your router's configuration interface.