The iPhone 4 has an amazing battery life. This is thanks to a design that allows for the internal battery to make up most of the iPhone 4's innards, as seen in iFixIt's iPhone 4 teardown (Image below):
Photo from iFixIt
Still, even this monster battery must give away at some point. Here are two that will knock your battery out in a matter of hours, so watch out. After all, you don't want to be without a phone in an emergency.
GPS navigation – This is the killer of all smartphone batteries. Many of us have used GPS navigation in the past, and its not unusual to use such a feature while traveling far away from home or driving through an unfamiliar city. Still, a roadtrip is also when you may need to rely on your phone the most. Be prepared to have a battery pack, or better yet, a car charger to keep your phone alive for when you need it most.
SKYPE (iOS4 Update) – If you are ever looking for a quick way to off your battery for whatever reason, try running Skype in the background. Otherwise, it may be best to quit the App when it won't be in use. However, if you must keep Skype on, and running in the background, we suggest also carrying around a battery pack and/or charger. In a realistic situation, you probably just need enough juice to get you through a typical work day, and a battery pack will do.
All of these are easy to find online. Sure, battery packs look a bit silly while attached to your phone, but the extra hours battery life will come in handy. We also suggest car chargers or even a backup home charger for your iDevice in case anything ever happens to yours. Of course, the latter doesn't become much of a problem if you have more than one iDevice since Apple has been pretty good about keeping the ports on their devices the same.
Here are the results for a simple search for battery packs and extenders for iPhone: Link (Remember to always check your device's compatibility with the product you intend to purchase and read what other users have to say about the functionality)
Here are some iDevice car chargers as well: Link
Need more tips on how to save your battery? Read our guide here.