Ran across an article on To-Done today that provides some helpful tips on how to say "No" when you need to. At first glance, this seems kind of like the anti-matter version of the book "Getting To Yes," but it's actually very complementary.
The core of this is taking responsibility for your commitments - before you make them. I'm sometimes guilty of taking on too much, of saying "Yes" a bit too quickly, and of avoiding "No" just to keep the peace. I usually realize it soon after I've dug my hole a little deeper, and then recovery can be a pain.
The list of ideas provided is great - now, if I can just get them to be more of a habit...