A bit of history, I've always been good with women when it comes to hooking up. While I certainly don't consider myself great at picking women up in random locals, I'm confident enough and good enough looking that I get laid and have them coming back for more. That is, they come back for more until I get interested and then they flake out.

I'm reasonable confident my issue is that with the girls I'm less interested in, I'm much more aloof and clearly give off the "you don't have this wrapped up" vibe. The opposite is true when I'm trying. To that end, I've tried to practice not coming across as too interested. This brings me to my questions, any advice on how to practice this?

Right now, I'm trying not to respond to her IMs immediately (forcing 3 points of contact before I respond). I've typically been the one to suggest plans. How do I know when is a good time to suggest vs build tension by holding off?

[EDIT] to clarify, I'm not trying to turn a hookup into a girlfriend. The girl I'm dating and I are both looking to the long term.

tl;dr - Strong with casual relationships. Bad moving to GF. How to make it work when you start caring.