To Make sure you're ready for said inevitable encounter, I've provided you with a great article I found on MSN.com.
Recently, former pop power couple Britney Spears and Justin Timberlake reunited during rehearsals for Madonna's concert in Madison Square Garden. The exchange between the old flames was said to be short and sweet. According to Debbie Magids, Ph.D., these two got it right. "When you run into an ex, the goal is to be as gracious but brief as possible," she says. Since not all ex-encounters are created equal, Cosmo identifies a few scenarios to prep you for your own potentially awkward moment.
When You're the Dumpee
If you bump into the guy who emotionally gutted you, put on a brave face. This means no yelling, blaming, or crying — these antics are best saved for your friends. And you don't want to confirm how smart he was to ditch you. No matter how poorly you're holding up, when he asks how you've been, smile and say, "Great!" Then move on.
When You're the Dumper
Guys behave differently than girls do when they've been hurt. Whereas it might be within your emotional capacity to "act" warmly toward the jerk who ran your heart through the shredder, when you're the jerk, don't expect the same treatment. Be prepared for his cold shoulder and don't dwell on it. Similarly, restrain yourself from being overly nice or concerned. This is only a means to alleviate your own guilt, and might even lead him on.
When You're Still Tempted
You kinda secretly hoped you'd run into him this way: out with your friends, looking and feeling gorgeous. You've had a few drinks and suddenly … you're missing him. Stop, take a deep breath, and mentally list all the reasons why the two of you were not meant to be. Then, remind yourself how long it took to get to this good place you're in now. Don't waste that kind of confidence and energy on him — instead, channel it toward the newer, hotter guy across the room.