If you are interested in a research career
And someone asks you do something that does not directly help your research career
Just say No!
This is yet another critical survival skill that often goes unmentioned. Sometimes it is difficult to tell what helps your research career and what doesn't, but in the end, it's going to be the quality and quantity of your publications. Occasionally service to your community may give you a slight edge in the review process, but in general it's unlikely to make much of a difference. (While good writing may compensate for mediocre research, I've yet to see service do the same thing).
I think a lot of people are afraid that if they don't say "Yes" to everything people will think poorly of them. In my experience this has not been the case. In fact, if anything, it reflects that one is a mature researcher who knows their limits and can manage their time well.
Research takes time - uninterrupted blocks of time where you can think, write, and do. If you are interrupted frequently it's hard to gain traction while at the office and tempting to procrastinate. But the "work at night/weekends" solution has its own set of distractions that also make finding uninterrupted blocks of time difficult (housework, family, blogging :-), pets).
To help learn to say oh-nay, I will share one of my favorite Sesame Street videos, in all its 1980s splendor: