You see your ex across a crowded room, and your first instinct is to run and hide. Don't. You can flirt with your ex instead, making it clear that the worst thing he ever did in his life was let you go. He'll be pining for you for months if you follow some simple tips.
Smile. The first thing you need to get across is how perfectly happy you are without him. Once you see him on the scene, put on a big grin and walk over confidently and enthusiastically to say hello.
Bring up some good times. Start the flirting by casually mentioning some of the best times you had together. "Hey, I was just thinking about how much fun we used to have river rafting. Have you been lately?"
Make him regret that you guys broke up. Bring up other aspects of the relationship that will make him realize what a great girlfriend you were, like your adventurous side. "In addition to river rafting, I've taken up skydiving and hunting. We could have so much fun if you ever came with me."
Make him jealous and sad. Here's where you let him know that you are having so much fun--and that he would be, too, if you guys had stayed together. Throw in a compliment while you're at it. "I'd invite you to my next skydiving lesson; your massive shoulder muscles would be perfect for it, but I'm bringing Bob again. He loves when we hold hands and fall through the air."