This one has always been one of my favorites:
A doctor and a lawyer were attending a cocktail party when the doctor was approached by a man who asked advice on how to handle his ulcer. The doctor mumbled some medical advice, then turned to the lawyer and grumbled, "I hate it when people seek free advice from me at parties."
The lawyer replied, "I know just what you mean. It happens to me all the time."
"How do you handle it," asked the doctor. "It seems rude not to answer a question when you are asked for advice during a social function."
"I just send the person a bill for your time" replied the lawyer.
"That's good," said the doctor. "I'll have to remember that."
The doctor went home and thought about writing a bill to the man who asked about the ulcer, but when he woke up the next day it had already slipped his mind. The night's events came rushing back to him the next day, however, when he opened his mail. In it, he found a bill for $100 from the lawyer for "consultation services rendered."