Uh, well. I've never had much a problem staying civil with an ex, but then I've only had one and I'm still really good friends with him, so. Just be nice and polite, though. If you don't think you can do that, then simply avoid him as much as possible and when you do have to see him just keep the conversation short and move along.
I used to leave bars or not go to bars she was at and then I just said screw it. She has a new boyfriend who's actually pretty cool so I just stay where I am now and smile a lot. Hope she doesn't screw him over in the end like she did to me.