There's been a general rule of thumb I've been following this past year that has helped a lot: don't do things you don't want to do. Following this rule has given me more time and productivity, less stress, and better relationships.
Of course, like all rules of thumb, there are certain exceptions. For instance, I believe we should go out of our way for those important to us or step outside of our comfort zone sometimes to grow.
If you find yourself saying 'yes' when you know you should really be saying 'no', think about all that is gained by saying 'no'. It helps.