"No" is a simple two-letter word, but it's one of the hardest to say to others. This is especially true when the other person is a friend or family member. You are often torn between saying "no" for your own well-being or agreeing to something because you feel obligated or are afraid that the other person will make you feel guilty. The truth is that you have a right to self-care and that no one can make you feel a certain way unless you allow it.
Say "no" immediately. When someone close to you makes a request, even if you know that "no" is the best answer, you may waffle and say, "Well, maybe. Let me think about it for a minute." This opens the door to discussion and leaves the possibility of a "yes." If you need to say "no," just say it outright.
Refuse to apologize for saying "no." There is no need to be sorry. The other person made a request, and you declined it as you have a right to do. Saying you are sorry implies fault or responsibility on your part. Simply say "no" and leave it at that. If you habitually say that you're sorry, practice saying "no" and stopping there.
Refuse to give an explanation. When you say "no," you may feel the need to add "because I'll be busy that morning" or some other explanation. This give the other person an opportunity to say, "Oh, but if you're not busy in the afternoon, you can do it then." You don't owe anyone a justification when you refuse a request, so don't open the door to further discussion and negotiation by giving one.
Be assertive and keep repeating "no" if the other person pushes for an explanation or tries to get you to change your answer. Don't let her push you into negotiation with a statement like, "If you can't help me tomorrow, when can you do it?" Simply respond, "I said 'no,' and that means I won't be able to do it at all." If she pushes you to explain, say, "I don't want to go into an explanation. I've already given my answer, so please respect it."
Don't let the other person make you feel guilty. If he says, "Well, if you don't watch my son, I won't be able to go to the job interview," don't take his problem on board. His son is his responsibility, not yours, and it's up to him to find suitable arrangements. You don't have to feel guilty if you can't assist. Simply say, "It sounds like you're going to have to find someone else to watch your son while you go to the interview." This restates the situation in a way that shows it as the other person's responsibility.