Here are a few of the techniques that I've seen employed successfully.
Make sure that every chapter of your story has moved your readers closer to the ending. Sometimes in big ways, sometimes small, but they should feel the wind in their hair as they read, even if they don't know where they will end up. Even if you don't know at the beginning exactly where you want to end up, once you figure it out go back over each chapter and ask "how does this chapter move me toward my climax?" If it doesn't, cut it, no matter how much you love it.
Make sure that the protagonist deserves what happens in the end. Readers need to feel that it was the actions of the protagonist that caused the ending.
There must be something unexpected in the end. If we all see it coming, the ending becomes quite dull. However, it can't be completely unexpected, because that feels like a betrayal of writer contract.
The ending of a story should sit firmly on the foundation that the story laid. For example, if a superhero arrives to defeat the villain completely out of the blue while the protagonist looks on, that's what is considered a deus ex machina. It's very unsatisfying and feels like a cheat. But if we can look back and see how the arrival of the superhero was hinted at (even if we didn't recognize it at the time) and caused by the protagonist's efforts (either deliberately or inadvertently) throughout the book, the ending becomes more satisfying.
They need to feel that they could have seen it coming if they had just paid a little more attention, but didn't.
Be aware of what your conflict is, and make sure it is resolved. If there are minor conflicts and you want them to be kept in "reserve" for a future novel, then at least resolve the immediate crises but in doing so create others for later.
Keep your promises. This is a little more difficult to define, as each reader will have different expectations. Critiquers are invaluable in figuring out what expectations your writing has set up. They may be as subtle as the "mood" of a story (if your world is set up to be a gritty, unsettling world with horrific abuses happening everywhere, your readers aren't going to be satisfied with a fluffy pink ending). They often have to do with the fates of the characters. If readers like a character, they are not going to be happy with a casual death. If beloved characters die, their deaths need to mean something. If you make your villain an admirable person, readers won't be satisfied with a simple defeat. Perhaps it would be better to reform him. Or at the very least give him a fatal flaw that makes his downfall inevitable.
Go back to the beginning. A lot of people say this, but few are able to define it well. You should be able to hear the echoes of the beginning in the end, and the end in the beginning. For me, this means I go back to my beginning and ask "how is this setting us up for the climax?" and of the climax I ask "does this make me recall the beginning of the story?"
Some of the best, most satisfying endings come when all seems lost and the protagonist is able to turn the tide. This shouldn't be a haphazard case of stumbling into the Great Solution or an accidental happening. I like to be able to look back and see that if anyone else had been in his situation all would have been lost, but because the hero is who he is, he was able to win through.