Try closing out of excessive apps by entering the multitasking menu, this can be done by double tapping the home button ( the circle button on the face of the device). Hold your finger on the apps in the menu at the bottom of the screen until you see small red signs. This will close the apps. Also if this doesnt help try restoring the device.
First, try to reset the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose data by resetting.
If you are using Push service to get your mail, you may also try to delete the mail account, restart the phone and add it back again. There have been cases in the past, where mail did not stop to connect to the mail server over and over again, causing the drain of the battery.
If this does not help, set it up as new device again: