In addition to Shirlock Homes excellent answer, you should also check down stream of the circuit load if there exist any GFI outlets ahead of this.  I have a GFI outlet on the Load of another GFI outlet then they can cause each other to trip.  If this is the case, you can simply replace the downstream GFI outlet with a standard outlet as it will already be GFI protected anyway.

Again things to check:

 1. Short or ground fault down the circuit

 2. Reversed polarity on receptacles downstream

 3. GFI outlets down the circuit.