In Vampire the Masquerade: Bloodlines when playing as a Tremere, I usually keep using Blood Purge on enemies (stops them and forces them to throw up blood, therefore stopping them attacking you) so that they have no chance of attacking me at all. It's not cheating, since it's within the actual game, but it is a little underhanded.
Also in Oblivion, by using enough items that reflect damage and magic you can acquire in the game, you can effectively make your character immortal to all but archers and falling long distances.