If you think you’re suffering from unusually short battery life, than try this Simple tips. Here is what you can do:

Optimize Your Settings
Lock Your iPhone
Yes this will really help you to save the iPhone Battery. To lock iPhone, press the Sleep/Wake button. You can also set the Auto-Lock interval so your iPhone will turn off more quickly after a period of inactivity. To set Auto-Lock, go to Settings > General > Auto-Lock and set the interval to a short time, such as 1 minute.
Use iPhone Regularly
For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).






Here is a list of more iPhone battery saving tips that I found interesting Great tips for using your new 3G iPhone I like the point about being able to download images from Mail or Safari.
Apple has theirs also iPhone Tips & Tricks but it feels a lot drier and more of a manual.