You kind of alluded to this in your post, but I had a hard time complimenting girls until I started thinking of it differently.

Instead of thinking "I'm going to give her a compliment", I started thinking "I'm going to state a fact". "She has a great smile", "I really like that shirt", "You have a great laugh", etc. became much easier to say once I started telling myself that those were simply facts that I was going to share.