Baby Daddy Drama

I need some advice on how to deal with my ex husband.  We were married for 7 yrs and seperated 2 of them.  He ofcourse does not pay child support like he should except when he thinks he has a chance of getting back with me.  First of that is pretty f'd up if you ask me.  He doesn't support his child unless he is getting something out of it.  I couldn't imagine not taking care of my children.  If anyone else has baby daddy drama leave me some comments or advice please!