Okay, well that's something we can work with. You have to make sure she comes every time. Honestly, it should be multiple times unless it's a quickie, but baby steps. Her orgasm has to be just as important as yours. And hers needs to happen first. She gets off (preferably several times) and then you get off. 

You want her to keep coming back for more? You want her to initiate more often? Gotta make it worth her while, my dude. You gotta rock her world every time. Part of this will just be paying attention to what she likes and doesn't, but a big part is communicating with her. Asking her specific questions about what she prefers and what gets her off will go a long way. Have her take care of herself in front of you and pay attention to what she does. 

There are tons of online resources out there that can give you a lot more information than I can, but that's where I'd start. There's a good chance you'll get some more often if you start upping your game a bit and make sure she never leaves a sexual encounter frustrated.