Oh... you are interested in a relationship. That's not my thing, but it still applies with some changes.
First, you have to be worth keeping... never get complacent. Just because it is your turn with a woman does not mean you can become a couch potato. You have to continue to be a guy that is doing and going places that cares about his body and how he looks.
Second, you have to let her know you love her, but she has to believe that you will leave if you are pushed to far. You do this by not becoming too compliant. This is really simple to do, just always put yourself first. There is a fine line between selfishness and putting yourself first. When she asks you to do stuff for her, turn it sexual and ask what she is willing to do for it.
Third, never abandon what you love, she will try to turn you into something other women hate because she thinks if you can 'change' you no women will want you.... she will say that she is only trying to make you a 'better person', in reality all she is doing is making you less attractive to other women. The problem is once you agree to this BS.. she starts losing attraction for you. She sows the seeds for her own dissatisfaction.
Okay... here is how it works... She cares A LOT what her friends think. She has to hear from them every now and again how 'lucky' she is to have you, if she does not get this re-enforcement from her friends then she will believe she can do better. This will make her fearful of losing you because you might be too good, so she'll start feeding you cheese cake and making you a slob.... which kills her attraction. You have to keep being an awesome guy, and keep her friends jealous, if she learns that she can't turn you into something other women don't want, she will be forced to work hard to keep you.