This is a Community Contribution from Jon, a GTD enthusiast who hails from the midwestern U.S.
From long To Do lists to overcommitted schedules, we tend to take on too much. When is the last time you said no to someone when they asked you for something? It can be hard to do.
Most of us want to help others when they need it. There are times, however, when we need to say no. I know I don’t like to say no. I like to help people. It feels good when someone wants you to do something for them.
It may help to start weighing that commitment against what you’re trying to accomplish in other facets of your life. 
Here are three questions to ask yourself when faced with saying yes or no:
1. Do you have the capacity to say yes? If you have the capacity, great, go for it. Say yes. Make sure you can commit 100% though. Committing and not delivering is much worse than not committing at all. You’ll know if you have capacity because with GTD, you already have an inventory of your projects and actions, the things you’re already commited to.
2. If you say no, do you know how to do it eloquently? Saying no could seem harsh on the surface, so sometimes you need to explain why. An example is simply saying, “I have a lot on my plate and I don’t think I can give you 100% of my attention right now. I’d love to help in the future if you need me.” There’s not a lot someone can say to that. You’re being honest. Most people will respect that.
3. Are you the right person for the job? Just because someone asks you for help doesn’t mean you have the ability to help them. If you have no idea how to fix your mom’s dishwasher would you commit to fixing it? I don’t know the first thing about fixing dishwashers. (My expertise pretty much stops at changing light bulbs.) Telling my mom that I can help would be doing her a disservice.
Same goes for work. Your boss asks you to compile some information on an upcoming project. He tells you he needs it by tomorrow morning. You’ve got three other projects you’re working on that are equally important. Do you have the courage to tell your boss no? Again, explaining how you can’t give 100% to it and offering a different solution will help.
Knowing when to say yes or no is a big part of stress-free productivity.
Although these are good questions, to me it seems they are asked in the wrong order, and the question “is this something I would want to do?” is missing. I would first ask myself question 3 “am I the right person for this job?”, then if this is something I would want to do (which can be tied to if you are the right person for the job: if it is something you really hate, probably there is someone to be found who doesn’t and is therefore more capable of the task at hand), then the capacity-question, and at last I would address how to say no, if I want to say no. If you are afraid of saying no or don’t know how to, that is a skill you should work on: not a reason to say yes.
In #2, Saying No Eloquently, I don’t think the example you gave would generally get much respect. I had a co-worker answer me almost exactly like that once, and while it was better than a simple “no”, it still felt like some kind of standard response, and like being blown off. I felt that if that person came to me with the same kind of request, I would have given them at least some more attention regardless of how busy I was.
I would say to at least provide some kind of explanation of what is commanding so much of your attention that you can’t be bothered to even give a small amount of thought about whatever request was made. Maybe then the person making the help request will be able to understand a little about how and why you are overwhelmed, and then you’ll have a chance at getting that respect you mentioned. Might be subtle, but makes a big difference if you really care about what the other person thinks.