How do you quickly find the Greatest Common Factor?

Lets say I want to reduce a fraction to it's lowest terms.

For instance

__14__

42

OK that one is easy, all I have to do is write out al the factors like this;

Factors of 14 = 1,2,7,**14**

Factors of 42 = 1,2,3,6,7,**14**

We can see that 14 is the greatest factor common to both thhe numerator and the denominator. I then divide the numerator and the denominator by the highest common factor like this;

__14__ / 14 = __1__

42 / 14 = 3

Which gives me my answer. So that one was straightforward. The problem comes when I am faced with something like;

__240__

165

Are there any tricks that enable you to quickly find out the greatest common factor of larger numbers?