When you bluff, you have to tell a very good story, if you bluff on a flop like AJ3, you'll have to tell your opponent, till the river, that you have at least two pair on this board. But bluffing against a made hand is not that hard, let me give you an example, on this same board, AJ3, lets suppose he's got QJ and hit his medium pair with a Queen there as a kicker, if you bet hard on him, he WILL have to fold, it doesn't matter what he has, unless he has a crystal ball on his hand or are those guys who thinks everyone is bluffing him.
But in case you see the guy wont fold to your bluffs, just forget that hand, unless you have a made hand in that case. For example, if you got nothing on a board like KT7, and you cbet on flop, he pays, the turn comes 6, you bet on turn he pays, on the river he will check to you expecting you go all in and catches you with a better hand than yours, or he will donkey bet telling everything he has