A time may come when your iPhone behaves so badly that even the fixes and preventative tips that I've offered elsewhere fail to resolve the matter. Such serious symptoms include:
Frequent and repeated crashes or freezes that are not remedied by restarting.
Failure of the iPhone to restart properly. Instead of going to the Home screen, it halts at the Apple logo or restarts itself again (and again, ad infinitum).
The Mac OS X single user mode screen appears at startup. It's a black screen with white text, and it is closely related to the screen you can access on a Mac by holding down Command-S at startup.
The iPhone screen stays black, failing to respond to any input even after allowing sufficient time for the battery to ...