Question: However, I am getting too many invitation these days and I need to start saying "No" in order to get work done. However, I am lacking clear rules except for "A dominates B in time-slot C". Any ideas from your side?
Answer 1: I pretty much say 'no' if
a) conference is too unimportant
b) people don't pay for my travel (invitation on my own account are cheap :)
c) I don't see how I would profit from attending this conference at all
d) I simply want to get work done
A good thing is also to promote good students to give talks for you, e.g., at workshops/seminar/etc.