If you are talking about fractions made up of complex numbers, such as (a + bi)/(c + di), the first thing to do is multiply the numerator and denominator by the complex conjugate of the denominator. This will give you a real denominator, as follows.
(a + bi)(c - di)/[(c + di)(c - di)] =(a + bi)(c -di)/(c2 - d2)
The go on with what ever simplification you can see in the numerator.
If you don't mean complex numbers, but complicated fractions with real numbers, then the answer will require you to give some specific examples of what troubles you.