1. Disable power-hungry features such as Wi-Fi, Notifications and Location Services.
2. Buy a battery pack, especially if you plan on taking your iPhone to places that don't have a ready power outlet like, say, a golf course. (Check out my .)
3. Get it tested by an Apple Genius, because anecdotal evidence suggests there are a lot of with poorly performing batteries on the market.
Eventually, though, you'll need to replace your iPhone battery.