To come up with a greedy algorithm quickly (or slowly) is easy. The difficult part is proof of correctness. My strategy is to prove by contradiction so that requires me to quickly think of a counter-example. The proofs explained in the CLRS book for some of the algorithms could be a good start. Proofs by induction are also good but to come up with them might get difficult. Bottom line is that greedy algorithms need to be protected by proofs.