Miss Manners: My friends keep canceling our plans at the last minute

Dear Miss Manners: What is the correct way to deal with being stood up for social plans?

Over the years, people have said things to me such as, “I’m going to break off our lunch plans for today, OK? I’m going to lunch with someone else.” Or, “I can’t meet you for breakfast on Saturday like we’d planned; I’m going to go and play golf with Dolores.” Or, at the last minute, “I can’t go to see the play with you this evening. My boyfriend says we’re going someplace else.”