Tell him he looks good. 

Find something about his appearance that you really like and point it out to him. His nice lips, sexy legs, or friendly smile.

Compliment his skills/technique in bed. 

It seems to be general knowledge that men love when a woman gives them sexual compliments. Some women are shy about this, but they shouldn't be, a great deal of a man's sexual satisfaction comes from knowing that he is satisfying his partner.

If he works out compliment his great posture. 

Or even better - if he has great posture ask him if he works out. Compliment his muscle control, his endurance, etc. Be careful, it's easy to go too far with this, don't just say things to flatter him, and don't go on and on about how big his muscles are. A simple comment that you're impressed by how well he does his exercises is fine.

Ask him for help. 

If he is at the gym doing the leg press and you don't know how to use it, go ask him to show you. If your car hasn't been working, ask if he would look at it. The point here is that he gets to be the expert, by helping you he feels smart and competent, and complimented.

Compliment his relationship skills. 

This is great because if you say something like: "I really like when you hug me for no reason" he will feel happy because you are pleased by him, and so he will do more of what he knows pleases you. You both benefit from this. Compliment his listening skills, how understanding he is, how generous he is, take notice of all the sweet romantic things he does for you, etc