I just had to do something I really didn't want to do, but after deliberating with it for a while and taking a realistic look at my time, I knew I had to.
I had to let someone know that I would be unable to work on their project. Now this was paid work and not just a volunteer effort or work on an open source project. Originally, I said that I could do it and we worked out a tentative schedule for when I would start and when I thought I would have an initial version to look at.
However, I had to accept the reality that I would be unable to start or even complete this project. I realized this after looking at my work schedule and my upcoming Summer schedule, that I just did not have the time. At least not without sacrificing time that I could be spending with my family.
Also, I have my own projects that have been put aside for way too long because I was always working on something else for someone else. At some point you have to just step back and ask yourself if you really need to take on another project and if you are, what you are willing to trade for doing so. It would also be a good idea to keep this in mind when initially presented with something to work on.
It is okay to say No and not take on more work.