People bluff when there is no other way to win a hand (or at least they think so). The problem is, they often bluff *because* there is no other way to win the hand. This is not a great strategy because it will not work too often. When you bluff, you have to try to point down your opponent's possible hands and think about what they would fold. You have to also consider the population tendencies. For example, in microstakes it's harder to make people fold, than on higher stakes, and it's especially hard to make people fold anything stronger than a pair. You have to also consider some people will call with pretty much anything: middle pair, bottom pair, ace high, king high. Bluffing them is close to impossible.
Assuming that you're not playing against a calling station and you can get them to fold, you have to consider what hand could they have and what should be the bet size to make them fold it. For example on a draw heavy board, if you think they missed their draw, they may end up with a garbage hand, and then they would probably fold to pretty much any sizeable bet. Middle or bottom pair is a bit tougher cause they have some showdown value, so you have to do at least 3/4ths pot bet. For anything better (top pair, set etc) you have to probably do an overbet or shove, with a caveat that in certain situations your opponent will never fold a set or better.