This is too funny!!
How about this. First off, STOP WORRYING SO MUCH!!
Ever sit on the toilet and push ya turds out? Hurts huh. How about when you sit down and things just flow smoothly? After you get done you feel very relaxed and fulfilled. This is a crazy analogy, but if you think about it, it makes complete sense. When you force a conversation, it's harder, but when you're laid back and carefree, it just flows with almost no effort at all.
If you still find this hard to do. Take a drink or two before you converse. Don't get drunk. Just get a nice buzz going on. I'm just this type of person, but when I have a drink or two, the world is mine and anyone around me is my pawn. They're here to make me happy, not the other way around. Everything I say is golden, if you don't think so, then to hell with you.
Stories will make you golden. The trick is to make it short and sweet. Build it up, and get to the point quickly. People make the habit of dragging on and on and on, and by the time you get to the end, it's not funny. Don't have a funny story to tell, make one up. If the story didn't happen to you, change the characters so it did. The more personal the better.
Overall, I would have to agree with Rex Man. Stop sucking so much! The day I sit around wondering how to make my ex laugh is the day I'll check into a mental clinic because I have clearly lost it.