"I have been busy"

Everybody has heard this excuse for an unaccomplished task: "I've been busy." I would like to make a plea for not using that phrase ever again. First of all, it's not an excuse for not doing something, it just means exactly the same as: "I haven't done that thing." OK, not exactly the same. There is always the possibility that you weren't busy but you were just bored to death, but you still didn't do that thing. Let's assume that that was not the case (and if it was, you wouldn't want to admit it). So saying you've been busy is (for most people) a non-statement.
I'm always busy, and I bet you are always busy too. Busy working, busy calling customers, busy spending quality time with my family or even busy sleeping, which is absolutely necessary if I want to spend the next day being busy and productive. Come to think of it. Perhaps we should just ban the word altogether.

So let's look at some alternatives that are more honest or more meaningful. And then, let's find out how if these alternatives can be made to sound socially acceptable.

I had more important (or: urgent) things to do.
That's honest. It's probably true, too. Depending on who you are talking to, you may need to convince them how important those things were, and that you'll get to their task as soon as your important things have been taken care of. Be careful not to make this sound like the other person is not important!
I was really busy working on a project with a big deadline next Friday. After that, I'll get to your task.

I had planned on doing this task tomorrow.
Yeah right! Try to avoid this one. If you had planned to do it tomorrow, you probably should have informed the other guy three weeks ago that is would lay on your desk for three weeks before you'd look at it. If it's just an excuse (which it probably is), then (a) make sure you have it done by tomorrow, and (b) mind your reputation and avoid using the same excuse twice.
It slipped my mind, but I'll have the report ready tomorrow.

Your task (or whatever) is on a low priority for me now.
Honest. But not very helpful. However, if that's the way it is, you should really get this message across in a way that the other person would know what to expect. This puts him in a position of uncertainty, akin to the halting problem.
As long as I we are struggling with quality assurance, I can't make a priority of this thing.

I'm not planning on doing it at all.
Good for you! You've just freed yourself of all guilt and all future reminders and requests to complete this task. Downside: depending on your relation with the other person and the nature of your task, you may need to
I'm not going to mob the floors any more, but instead I'll make arrangements to get a housekeeper. (or: I'll buy an iRobot)

Best solution is to be very clear to other people on what you want to commit to, and by what time you can deliver. Also let people know what the implications are when you agree to a task.

• I'll mob the floors tomorrow, but then I won't have time to go grocery shopping.
• I can have your report ready by next Friday, but that means the other project you assigned to me will be delayed.

Everybody knows that you are busy. Just be busy doing the right things.