Getting to No
A lot of people find it hard to say No. They end up overburdened with work, harassed and feeling resentful at others for asking, and at themselves for agreeing. Usually ten minutes after agreeing they find themselves boiling with anger at their own stupidity, or depressed about letting themselves be walked over again.
There are no good ways of learning to say No: but there are nany good ways of Learning to Not say Yes.
a) Need to check. Get yourself time to think. Tell the person asking that you will need to consult your diary / your husband / your schedule - or something else first before you will be able to agree. Tell the person you agree that it is important, that it needs done, but you cannot commit to it right now without information from somewhere else.
b) Change the Format. Put off answering until you can reply in a different format. Many people are embarrassed by saying No face to face. It is much easier Not-to-say-Yes by email or a written note. Or get someone else to tell them. It is much easier to give an excuse when you don't have to talk to them directly.
c) Invent a boss. Tell the person that you need to get agreement from someone else. It can be a purely fictitious person. Tell the door to door salesman that you need to consult your aged mother who has all the money. Tell a difficult customer that you need to talk to your supervisor first, and they are not in. Email back to a request by including a false email from your 'boss' quoting that you are not allowed to do what they are asking.
d) Make up a rule. Tell them 'I never make a decision without sleeping on it first. Your idea sounds really good. I will get back to you tomorrow'. No one is going to say you are not allowed to have that rule so they will have to go along with it.
e) Turn it back on them. Tell them that you will do it: but they have to do something first.
f) Give them alternatives. Invent a commitment. Say that the best time to do what they want is this afternoon at three, but you have something planned for then. Could they find someone to take over the three o'clock task for you?
g) Promise to delegate it. Tell them that you will ask someone else to do it for them. But that you will get back to them later to say whether that other person agreed or not.
h) Suggest someone else. Deflect the person by saying that you think some other person would be a much better choice, and say why. And then make them ask the person first before coming back to you, and if they do, suggest someone different.
There is no limit in how creative you can be in Learning-to-not-say-Yes.