Years ago I read a little book called "When I say No I Feel Guilty" that gave some of the same tips as your article. I am a person who can say no but then I feel guilty and second guess myself.
I think we all have to set boundaries and then stick to those. Decided what we will and won't do for others. I babysit some for my stepdaughter so she doesn't have to pay for daycare but I set limits on it. Otherwise, I'd be overwhelmed and angry. For my own sake, I have to set limits and know just what I'm willing to contribute to the daycare effort before it gets to be a burden.
I guess my tip would be to consider ahead of time what you will or won't do and then stick to it. That gives you a chance to know within yourself just why you are saying yes or no. It keeps you from being overwhelmed by over committing yourself.
