Recently, I've had quite a few friends asking me what to do when their iPhone crashes or locks up in a way that prevents the iPhone from responding to the normal iPhone restart procedure. This happens occasionally with jailbroken iDevices whenever there are cydia app conflicts with the firmware.
Update & Restore
Submitted by Steve Green on Thu, 04/09/2009 - 00:00
Resetting your iPhone takes it back to the way it was when it came out of the box. This can be handy if you’ve installed a bunch of apps and your home screen is a mess, or if you’ve misspelled words and the misspelling have been incorporated into your dictionary. To reset the device, simply tap on Settings in your Home screen and then navigate to General >Reset.
Submitted by Anonymous on Wed, 10/01/2008 - 15:44
You should always use the latest version of iPhone software on your device. You do this through iTunes and have two options: to update the device or to restore it.