Assuming your opponent is bluffing

[Haichan]

One of the biggest leak some players including myself seem to have is over estimating the number of times an opponent is bluffing you. Its especially a problem in the mid stages of a tournament when an opponent has been quite aggressive and is betting you on a reasonably safe flop where you have middle pair. I know I am too often assuming that the opponent has nothing or a draw and too often stay in hands that I should fold. Could even be a gut reaction that you hate being bullied and want to assume its a bluff. Anyone got any good advice on how to control the times you assume its a bluff.