How do I tactfully tell my advisor (who's much busier than I am) that I'm too busy to do such-and-such? I recognize my advisor is busier than I am. But, nonetheless, I feel the need to tell her sometimes that I am overloaded with too many tasks and I need to push a deadline back or pass on some request.

How do other people handle this tactfully?
I have had this discussion with my supervisors/advisors many times, all with positive outcomes.  This is what I do:


Be direct and to the point, it sounds like neither of you have time to dance around the issue.
Offer alternative ideas/timelines/strategies, I found that coming up with a solution is appreciated as in the long term, it is going to save time.
Be honest and truthful, state that you are overloaded - show how this is so, and either defer or decline the request.


Remember, your time and well being is just as valuable, and any good advisor already recognises that fact.