In the two final betting rounds you either maximize your winnings or minimize your losses. It's absolutely vital that you evaluate the strength of your hand at all times. If you think you have the best hand, your goal should obviously be to get as much money as possible out of your opponents. Adjust your betting strategy accordingly - how much money are the other players willing to risk at any given stage? Analyze how the hand has been played and try to put your opponents on likely holdings. If you think a player is on a strong draw you have to protect your hand by betting. Sometimes you should check a strong hand in order to induce bluffs from players who you think missed a draw, since they are not likely to call if you make a bet.
In no-limit games you should often bet small with the strongest hand. This type of bet will make players call with weaker hands. An alternative is to bet 80-120% of the pot. Your opponents will suspect a bluff, and this type of bet actually generates almost as many calls as a small one.
If you're in doubt about the strength of your hand, you have to be careful. Are your draws worth going all the way with? Do not call the hand down with just a medium holding, unless you're up against a habitual bluffer or a player who is quite obviously on tilt. Avoid betting unless you are quite sure to win a showdown, especially when facing strong players. You have little to win and a lot to lose (90% of the time you will only get calls from players who believe they will beat you).