Betting your hand properly can be all the difference between winning a lot of chips, winning a few chips, or losing all together. Remembering that any hand can lose and any hand can win, it is important to understand that betting can have a lot to do with it either way. If you push all your chips in pre-flop, there is a good chance you will win, no matter what the cards are, since most will fold. If you limp in every hand, there is a good chance you will allow others to hit better than your pocket aces. The key to betting is putting enough chips forward pre-flop to give yourself the best odds against a limited field, and to continue to bet your hand well enough. You do not want to give "free cards" to the other players to hit a better hand than yours.
How much to bet depends on the table. On a very tight table (few hands are played and very few raises made) a moderate raise may scare everyone off, while a moderate raise on a loose table (chips are flying everywhere) means a pot odds call to all nine other players. What you should pull from this is that limping with a big hand (AA, KK and even worse QQ and JJ) is a sure fire way to get yourself committed into a pot with several other players lowering your odds of winning. This usually will force you to fold or you may lose a lot of chips if you are unable to fold your pocket pair.
If for some reason you are lucky enough to win with the hand after limping in, usually the pot will be much smaller since you didn’t bother raising and nobody else wanted to either. At minimum, always raise at least double the blind, to put a hesitation in the back of the other players mind that you may have something they should be worried about. It is better to raise bigger and take the blinds than to allow your opponent to hit the flop hard or have a very good draw that has the odds to beat you if you push in. Winning some is always better than losing a lot.
Example: You have AdAh and limp in. The player on the button (last to act before the blinds) has 8c7c (c=clubs) and decides to limp in too. The flop comes As5c6c. This is a horrible situation for you. Even though you have hit trip Aces, the other player has 14 outs (See post on Outs in All About Odds).
In this example, the other player is actually favored to win the hand with 56%-58% to hit a straight or flush beating your trips. Yes you have a chance to hit a full house, but your odds for that are much lower. Limping in has created a problem which cannot be undone. Either you or your opponent are going to lose a lot of chips in this hand, and since you are no longer favored to win the hand, you will be the big loser 56% of the time (or more) in this situation. Trying to sucker or slow play with what is essentially a weak hand, a single pair, is actually a very poor play over the long run. Don't be greedy for the big pot, because that big pot containing most of your chips bet after the river, may be heading in the wrong direction due to the "free cards" and runner, runner straight he hit.
Over-betting your hand (betting too much, being greedy) is the same as bluffing. Sometimes it works, sometimes it doesn't, but at least you have something. The problem with "something" is that you can overrate your hand thinking it is stronger than it actually is, giving away your entire stack on a weak hand. Good rule of thumb is if you hit top pair and you don't believe the other player has a better hand, bet between half and the full amount of the pot. If they call or raise, re-evaluate whether you actually have the best hand or if they are fishing and have not hit a bigger one yet. Don't give them pot odds to call by making a small minimum bet.