For me, I would have to say never tell someone you are too busy. This is something that can be applied to both one’s personal and professional lives. The person who gave me the advice said using that excuse indicates you do not value the person making the request as a high priority.
How can loved ones, friends and customers not be high priorities? I admit I still slip up on that one once in a while, but when I do and use the “B” word, I immediately think back to that advice. My advice: Even if you really are too busy, still attempt to find a way to make the request happen.