How do you find the balance of coparenting without being nasty, rude, or without anger yet not be your ex's best friend?

I can't find that middle ground. Either I hate him and the thought of him makes me sick which makes me really rude to him or we act like buddies. Hang out, talk, joke, smile, have fun.

Where is that dang middle ground?

Me: 46 FWS: 36
Married and Divorced 4/07, Pregnant 7/07,False R 7/07
Baby Girl born 3/08
Kicked him out because OW: 7/08
5/10 He realized what he had and lost.
Moved home! REMARRIED 3/14/11!!