These days, I try to never tell people I’m busy.
It just feels more honest that way. When someone asks you to do something, and you reply “I’m busy,” what you are really saying is “I am doing something else that I consider to be a higher priority right now.” This is true by dint of the simple fact that if taking more time to talk to them or to do what you are being asked was a higher priority, you’d stop doing what you are doing as long as you felt necessary.
If it’s your priority to wrap up the thought you are writing, or the page you are reading, or the lap you are running, tell them that. “Let me finish this up in just a second/minute.” If you have a scheduling conflict, tell them you have a prior commitment you can’t move.
Sometimes, though, you just have to say “no.” It may be physically impossible – “Sorry sir, but I can’t get Jupiter moved closer by tomorrow night…”. It may be something that you have no interest in – “Sorry Bob, I’m not going to set aside a week to go to the snail darter convention.” Of course there’s the obvious caveat that just because you’re not interested, doesn’t mean you shouldn’t be.
Some people will dislike it, but compared to a vague “I’m busy” – letting people know when you are and aren’t available gives them a concrete idea of what your boundaries are, and a better idea of when they can actually expect something done. It also gives them an idea of how to get you interested. You see, going hand in hand with this is a little bit of tact. Being polite with anyone you are turning down is just good common sense unless you have a reason to be actively rude. I also try to not tell my clients “no” if I can avoid it. Unavoidable “no’s” include “You can’t install that desktop drive in this laptop case,” but many of them, even if it’s not your core business, are avoidable.
So how do I deal with “avoidable” no’s? Through “Yes, but.”
You know what your priorities are. Factor in what would make taking on a project worth your while like time spent researching, and opportunity costs. “I really was looking forward to the beach vacation, finishing that book, and on top of that I’d have to bump two clients a few days to get this done.” I’ve even factored in whether or not a client is particularly needy or requests more than average rework by reflecting the additional time it would take up on the estimate. For business, these are often relayed in dollar amounts or working conditions.
Either way, this puts the ball in the other person’s court. They now know what it will take to make something enough of a priority for you to tackle it, and they now have to decide yes, or no.