One big component of being a Don Juan (as opposed to being a player/PUA) is, that you don't need girls anymore. You just live your life, you have so much going on and only in the back of your mind you realize that suddenly all these girls start hitting on you. For me, this happened for the first time one and a half years ago, when I was so busy with school, new hobbies, partys, self-improvement etc, that I didn't think about seduction anymore and I started to seduce unconciously.
The problem is, that at some point you think: Fine, all those girls are hitting on me now, let's pick one for sex. That's what I did. I didn't have any interest in "spinning plates" because I simply hadn't got that much time for girls. So I picked one, had sex with her and immidiately I started to care about her. What then happens is, you start to think about her pretty often, stop not caring about girls which lowers your attraction and soon she's the only girl left so you care about her even more.
What is the solution to this dilemma? I'd say one way is to always have a backup chick (like pook said). This is easy and works really well, but the problem is, it will not help you if you are aiming for something more serious, because then you have to put some emotional investment into the relationship (and you should!). The only other way is to try and fail and eventually learn self-control, when you can care about her but still control emotions such as jealousy etc, so you can still live your life without wondering about her all the time when she's not with you.