I call this "Doyling" and I think it's probably the greatest sin in written word. Literally. It disgusts, confuses and annoys me.
Named after Sir Author Conan Doyle (or something like that) and his killing of, and later revival, of his character Sherlock Holmes. After killing him off fans demanded that he be revived; and so he was. The excuse was that the mysteries happened BEFORE his inexplicable dip into a waterfall, ergo, it wasn't cheating.
Doyling. Any time you revive a character that you didn't lay the groundwork out for PREVIOUS to the death is Doyling. If people can be revived, that's fine. Make it clear beforehand.
SPOILER ALERT.
Jesus, for instance, was not a Doyle. Jesus, before his death, revived Lazarus. Therefore, when comes back to life himself later on in the story, it's not impossible.
Doyling, noticibly, only occurs on likable characters and truly sinister villans. If you don't have a story without the death of the character you should probably rethink killing them.
More forms of Doyling:
Flashbacks: Yes, in the sense of context, you're not cheating. But if you kill a character off he should be good and dead and any more time spent with him is something you missed.
Notes from beyond the grave: Why does everyone who died in a parlor write out an exquisite not detailing, almost as if it were an outline for the story, characters and their true motivations? Couldn't they have just told the protagonist? Did they have to hide it from everyone but the hero? What is about being stabbed them makes someone so attracted to a stationary over a cellphone?
Spontaneous revival, or, the AH-HA: No. Either kill your character or don't. If you have them falling to death, don't make every character in the book so damn stupid that they can't look down the hole and say "Hey, this is only a few feet deep!" This is typically done with villains but can be done with good guys, too.
It only LOOKED like I was dead!: Also, no comatose states. Unless someone else is in a comatose state and revives out of it, or you've specifically introduced the vial of elixir that causes this state, don't. This rule is also know as a J&R or a reverse Romeo and Juliette.
I'll leave you with this: You're sad that your character died. Do you think your audience will feel the same way? If they do you've done your job as a storyteller. You've made them feel something genuine. If you're going to keep them around they damn well BETTER be able to pay more in whatever qualities you like than that tragic death. Otherwise you're just taking the suckers money.