Let's face it—batteries are Apple's only real downfall when it comes to their products, and the iPhone is no exception. If you're constantly recharging your iPhone's battery, it's probably time for a replacement (battery, not iPhone).
You can choose to hand it over to Apple, who will send it out to get fixed, leaving you iPhone-less for weeks, and charge you overpriced fees, OR you can buy a replacement battery and do it yourself. This video goes through the whole process of replacing your iPhone's battery, so pay attention and save some money.
NOTE: This is only for the Apple iPhone 1st Generation, not for the iPhone 3G, 3GS or 4.