IntoMobile has a few good tips on how to extend your iPhone's battery life, and most of them involve the most obvious thing you can do to keep your iPhone running: cut down on any and all extra functions. They actually recommend to turn off 3G, but you don't have to go that far -- just by holding down the Home button, you can close any background applications sucking up juice. And by resetting your phone, you can do the same thing -- clear out anything running that you're not using.
Actually, while I was in Los Angeles last week, I heard the great Leo Laporte mention a great tip on his radio show: turn off the "Ask to join networks" feature in the WiFi settings. If you're like me, you almost always know when there's a WiFi network around that you can use on your iPhone, and so it's pointless (and a waste of battery life) for the iPhone to be constantly searching for one. You can always flip it back on if you do want to do a little poking around, but leaving it off will significantly help battery life.
In fact, when in extreme trouble, you can go even farther and just switch the whole thing into Airplane Mode. It'll make your phone useless, but when you really need it -- out on a trip, or waiting to make an important call -- the extra battery time might make all the difference.
[via Apple Enthusiast]