The older your iPhone gets, the worse its battery performs. Don't blame Apple: it's just the nature of rechargeable batteries that they wear out over time.
Sadly, you can't just swap in a new one--not without some warranty-voiding surgery. But you can plug in external power, which is cheap, easy, and affordable.
For example, Meritline has an 1800mAh iPhone backup battery for $9.99 shipped. That's after applying coupon code MLC262531102717A (which expires Oct. 29) at checkout.
The standard iPhone 3G battery has an 1150mAh capacity, meaning this plug-in pack should afford at least 50 percent more runtime all by itself. You can use it for emergency power if your battery's on empty or to recharge your iPhone on the go.
And, hey, 10 bucks? Grab a couple. Throw one in the glove box, another in your coat pocket, etc. That way you'll always have backup power when you need it.
This battery pack is compatible with all iPhone and iPod Touch models. Keep in mind that because it ships from Hong Kong, it could take a couple weeks to reach your mailbox.