How do you keep the girl?

I consider myself to be a good looking guy….I am not flattering myself but I consider this because of my past experiences where I have been hit on a lot by girls from since I was 13 and also from what friends and relatives tell me. I have no problem in attracting girls, because most of the girls that I have dated made the first move on me first, but all of my relationships were short termed. I want to know how does guys keep the girl, how do I hold a long term relationship. My cousin, is a “pretty boy” like me and he has a successful relationship, but for me something always goes wrong…how do you guys do it?