How can I increase battery life with iOS 4?
Restart the iPhone (press and hold Home and the top button simultaneously till you see the slider to turn it off, and then slide the slider. Once off, press and hold the top button to turn it back on)
Turn off Bluetooth (Settings > General > Bluetooth) if you're not using it.
Turn off the WiFi option "Ask to join networks" (Settings > WiFi) to keep the WiFi radio from constantly scanning for new networks when you're not connected. It will still remember and auto-connect to networks that you've connected to before, but you'll have to manually connect to any new networks.
Turn "auto-brightness" off (Settings > Brightness) and set the brightness level lower. I would only do this as a last resort if nothing else seems to work.
Let the battery run all the way down to zero (i.e. let the phone run till it won't turn on anymore), and then recharge it all the way to full.