The irony is that I've consistently gotten more respect by saying no than saying yes. If you say yes all the time, you become the person people go to when they want to dump things on someone. When you say no when you're busy, people quickly learn to respect your time more, and they'll be far more grateful for your time when they get it.

More importantly, if it helps you deliver what you agree to deliver on time with more consistency, people quickly learn to appreciate that.