Be Prepared to Be Professional
Hurt, angry, guilty... All those feelings -- and temptations to have your revenge at, say, a staff meeting or at the water cooler - need to be kept in check. This is easier if you don't work closely together, but be prepared to be polite and professional even when you think it might kill you.
Control the Rumor Mill
If your coworkers were aware of the relationship, give them a vague update on the situation. No details on who-did-what-and-why and no bitter tirades against your ex's character, reputation or lack of bedroom prowess. Save all that for your friends. …