Unfortunately, modern large screen phones use quite a bit of juice so there's not too much you can do.
- turn screen brightness down
- reduce the time the screen stays on
- turn off sync and location services
- reduce the times things like e-mail sync
- turn off radios you're not using (WiFi, Bluetooth)
- turn GPS off when not in use
- turn off 3G when not using and there's no service or poor service
All these things may make a minor difference, but they're not going to dramatic.
You might want to consider either buying a spare battery or an external battery pack. The battery pack is probably more expensive but most will be able to re-charge your phone 2-3 times before they need to be re-charged. Search the forum for recommendations of which to buy.