When I started working, I was very eager to please. This meant that I had a hard time saying no to things people asked of me. I worked a lot of overtime, and still didn't finish everything that was asked of me. The result was disappointment from their side, and almost burning out on my part. If you never say no, your yes is worth very little. Commit to what you can handle, and if people keep asking you for more, make it very explicit that this would mean not doing something else. What I did was to have a list of stuff that I needed to do on a piece of paper with me. When someone asked for something, I showed them the list and asked what I should bump to have time to help them. This allowed me to say no in a nice way.
It's not easy saying 'No' and sticking to it. It can be so painful that we sometimes avoid it just to avoid short-term pain, only to find later that the long-term pain is even worse. We can feel mean for saying 'No' and so we offer reasons that justify our decision and soothe our bad feelings. But each reason you offer may be countered by a reasonable argument for taking the opposite position. And eventually you might surrender and change your decision to 'Yes'. If you can't say 'No' and stick to it, your 'Yes' is meaningless and it's gonna hurt.
Try using Satir's Soft Spurn to say 'No' without offering reasons and without feeling mean. First show appreciation, then give a regretful 'No' without any reasons, and make an opening for some future relationship. For example, "I'm flattered that you'd ask me. Unfortunately, I'm unable to do that at this time."