There have now been three generations of the iPhone, and while each one has gotten more efficient in its battery use, it's pretty common to be running short of juice after a day of mapping, photography, swarming, spelling or flight control-ing. Here's a roundup of the things I do to keep everything chugging along until the end of the day:
1. Disable “Ask to Join Networks”. Don't ever re-enable it. When you want to join one, enable wireless and choose one. Not only is this prompt annoying when you’re cruising between coffeehouses, but it feeds on your battery by constantly polling for new Wi-Fi signals.
HowTo: Tap Settings –> Wi-Fi, then flip “Ask to Join Networks” to Off.
2. Disable Wi-Fi. If you're away from home, you're probably not going to need it. The trouble is that the iPhone is constantly scanning for networks to connect to, which uses an incredible amount of battery. When you're around a router, you can re-enable it with three easy taps.
HowTo: Tap Settings –> Wi-Fi, then flip Wi-Fi to Off.
3. Enable push email, where possible. Gmail doesn't do this by default - you have to set up Google Mobile Sync. I've been using the excellent Gmail-for-mobile webapp since it supports search, labels and offline composing – just go to gmail.com from your phone, and add the bookmark to your home screen. If you'd like a popup notification for new messages, check out Boxcar.
4. Disable email polling. This stops your phone from checking for new messages every few minutes. Even leaving this setting at "Every Hour" will reduce your battery life by a noticeable margin.
HowTo: Tap Settings –> Mail, Contacts, Calendars –> Fetch New Data, then change the fetch interval to “Manually”.
5. Turn the screen off as soon as you're done looking at it. Your screen requires more power than anything else, so hit the sleep button to flip it off instead of waiting for it to automatically happen.
HowTo: Just tap the sleep button. Magic!
Bonus Tip: Thanks to @billtalco for suggesting that the Mophie Juice Pack can extend your battery life, if you're willing to shell out for an accessory.