The Power of a Positive No by William Ury (Amazon) is really an excellent book. Reasonably short read, and definitely worth reading. Recommend.
Essentially it lays out a framework for saying “no” that is in fact “Yes! No. Yes?” The first yes is the reason why you are saying no – no to that assignment, because your yes is commitment to quality? No to the overtime, because your yes is commitment to family time? This builds upon an idea from Stephen Covey’s 7 Habits of Highly Effective People (Amazon). The second yes, is the (hopefully win-win) proposal that you offer in the hopes of continuing, and even strengthening the relationship.
The only thing that irked me in this book was that it seemed that many of the examples involving women were disciplining children, or asking their husbands to work less. But despite that, I think it’s incredibly relevant – even with the extra issues women face saying no, or even because of them. Having a framework, being clear in commitments, is maybe extra necessary in that context.