My rules of thumb for pushing pre-flop:
1) Any time the pot has reached smallish medium sized always push. If I'm playing 100nl and the pot has reached 15 dollars I would push.
2) Any time I believe there is a good chance that somebody will call me I will push.

If I'm not pushing then I am at least raising enough to encourage most of the callers to fold.

Pushing post-flop:
1) The larger the pot is the more correct it is to push
2) The more people there are the less correct it is to push.

Generally, I believe it's much more dangerous and less profitable to push post-flop. It's not only inexperienced players that prefer to push pre-flop. Two of the best players in the world, Stewart Reuben and Bob Ciaffone, recommend pushing pre-flop in their book, Pot-Limit and No-Limit Poker. They don't push to steal blind but if there is alot of action before them with raising and reraising then they will push.

Whether you push post-flop depends completely on the circumstance. If I am behind the the original raiser and he raises on the flop I would push, considering the pot has probably reached a substantial size.

What you want to watch out for is the guy that didn't raise but only called the raises getting aggressive.

You should use your opponents previous starting hand selections to narrow down what they have.
our
I agree with the majority of the posters in this thread. A big multi-handed pot on the flop is exactly the situation you don't want to be in with pocket aces. If that's the situation that you find yourself in oftern then you are playing them wrong. The beauty of pocket aces is that you know you have the strongest hand pre-flop. The best possible situation you can be in with aces is pre-flop with lots of action because you have the option of pushing in your chips when you know you have the best of it. If you sacrifice this then you give up alot of +EV of the aces.

By letting a bunch of people into the pot you are giving everyone an opportunity to see whether they might connect with the flop. Every small pocket pair or suited connector gets to see the flop and fold if they don't connect or go all-in if they do.

If you don't want to go all-in pre-flop then fine. But reraise by greater amounts pre-flop to reduce the amount of people to two or less. If the pot is enormous and there are two people on the flop then it's and automatic all-in.