I've just read Jana Kemp's book, No! How One Simple Word Can Transform Your Life, and thought it might be helpful to mention since a number of people I have talked to recently have mentioned an inability to say No! or even just no.
First of all, its useful to remember that saying no to someone does not need to be you rejecting the person, being deliberately difficult or obstructive or trying to damage what the requester represents, be it a job, charity or friendship situation. Life can carry on after you have said no and people will not hate you for the saying if you do it politely, deliberately and firmly.
However if you say yes and don't mean it, that can ultimately get you into more bother when you don't follow through or your yes turns out to have been maybe at best and you start to lose repsect. Unless of course you manage to achieve everything you promised in a fit of superhuman power. And then collapse and pay the price yourself.
Throughout the book, Kemp talks about the Power of No, and she takes Power to stand for:
Purpose - what actually needs to be done here?
Options and resources - even if say no can I offer anything in return or make other suggestions
When - what is the actual deadline and if I say no now can I propose yes for a different deadline
Emotional Ties - how do you feel with yes or no and other feelings related to the request
Rights and Reponsibilities - what are your rights if you say yes or noe.g. rights to call on others to help, recquisition resources for the project
They are certainly topics worth considering before you unthinkingly say yes again but once you have done that you'll need to be assertive in the way you put across any no answer. Be clear, polite and if possible friendly and stick to your guns. If necessary repeat your bottom line until they get it.
The book goes on to look at the idea of self-defence; sometimes you need to say no to defend yourself, your time, health and position. There is also discussion on the ethics of saying yes or no, especially when you don't actually mean it.
I can imagine if you are really struggling to say no a lot of the time and it causes you stress that there are definitely helpful ideas in this book; you probably want to pick and choose which bits you read though.
At the end of the day I suspect, like all self-help type books, it might only take you so far and having someone external (partner, family member, friend or a coach) dedicated to helping you will reap far greater rewards. Get in touch if you want to know how a coach could help you in your particular situation.