Here is some advice on keeping a woman, courtesy of my friend Phil -- the best womanizer you or I will ever know!
He says you have to always hold something back, and always leave something up to her imagination to keep her curiosity alive. Most of us guys are pretty much the same, but women don't know that, and curiosity is one of your best weapons.
Another thing he said was that you have to make her miss you. Everyone here knows about scarcity and being unavailable, but in order for scarcity to be effective you have to be sure of one thing -- the time she does spend with you must be absolutely amazing, and without a doubt the best time she could have with anyone.
Now I'm not saying you have to take her to new places and show her new things. It does help, but it's not necessary. I can go to the mall with Phil and have the most awesome time ever. You need to be able to create an awesome, MEMORABLE experience with anyone, anywhere, especially when it really counts.
Also, she can NEVER be the first priority in your life. Always put her second to something, whether it be your family, career, friends, whatever. But leave a small piece of hope in her mind that she could become #1.
Plus, he says you have to make her put some work into hanging out with you in order for her to value you. Make her come and pick you up, make her spend some money on you, make her call and do you favors, etc. When she does things for you it will justify her own feelings for you and allow them to grow.
This is excellent advice no matter what stage of the relationship you are in.