Teri,
You're right,"exercising that 'no' muscle" is a must. Also I think one of the things we can try to do for ourselves and other women is positively support those that flex that "muscle" with words and actions that convey that we respect their decision to say "no" and that they should do so without feeling guilty. So many times I have said "no" to a project that I know I don't have time to do, only to be reeled back in by someone who refuses to hear my words and, often times without knowing it, has guilted me into participating. That "muscle" really needs to be in good shape to resist finding yourself in the middle of too much too fast.