This should be a no-brainer, guys: tell your wife or girlfriend she looks pretty, and tell her often.
I'm going to assume since you are in a relationship with this woman that you are, or once were, attracted to her. If you've been in the relationship for a long time, I'm going to assume that you have stopped complimenting her often, or perhaps altogether. This is a major oversight on your part, fella. We women need to be reassured of our attractiveness, and since we are no longer playing the field, the only place we can get that reassurance is from YOU.
"But," you say, "my wife has let herself go and she doesn't even try to look attractive anymore, so why should I encourage that?" *virtual slap upside the head* Do you realize that part of the reason she may have "let herself go" is because she feels like you didn't seem to notice or care if she made the effort?
I was out for drinks with a friend the other night, and this subject came up. The exact scenario I just described was going on in her marriage: she had always dressed nicely, worn makeup, and fixed her hair for her husband, but she recently stopped doing all that. Why? Because her husband didn't treat her any differently when she made that effort than he did when she frumped around the house. And why would she waste all that time and energy on her appearance if it wasn't necessary?
Truth be told, women have a hard time accepting compliments, and a lot of us will respond in a way that makes you think that it would have been better to say nothing at all. Don't let that discourage you. Though she may argue back with something like, "But I feel so fat today!" keep in mind that your words, more than likely, still gave her that little boost of confidence she was looking for.
Complimenting your wife or girlfriend will do more than just make her feel good: it will make her want to look good so that she can hear it more often. As an added bonus, it has been noted in many a research project that one of the biggest sexual turn-ons for women is having a partner who is turned on by her. Yes, that's right--if she knows you are attracted to her, she is more likely to be attracted to you. Could this be any easier?
So, to sum it up: if you want a partner who looks her best and wants to have sex with you often, keep this short but sweet phrase in your daily vocabulary: "You look pretty."