If your iPhone is exhibiting inordinately short battery life and you're not sure why, check your Mail settings.
First, if you're using a Yahoo! account, navigate to Settings > Mail then select your Yahoo! account and choose "Advanced." Check the "Use Push Mail" setting. If push mail -- which allows the iPhone to notify you of new mail as soon as it hits your inbox -- is enabled, it can put a serious hurt on your battery life.
For other email accounts, reducing the email check interval to every hour (In Settings, choose Mail then tap Auto-Check) or Manual has significantly increased battery life. If you are going to use a short mail-checking interval, tap the Sounds pane of Settings and turn the option for New Mail off.
For more battery saving tips, see our special report.