"Always give your real reason for not doing something."

For example, a friend wants you to join a fiction book group, but you just don't like reading novels. Don't say, "I'm sorry, I don't have a car on Thursday nights" -- your friend may figure out how to get you a ride, or perhaps offer to switch the group meeting to a different night. In which case, you either end up doing something you don't want or coming off as a jerk for not giving your real reason in the first place.