Are you scared of saying "no" to people? Are you worried that you'll look bad, not be liked, or come across as rude or selfish if you do?
Sometimes we don't want to say "no" because we think we'll lose a friend or we want to help everybody. But saying "no" doesn't mean you're rude or disagreeable. It also doesn't necessarily mean that there are going to be fights or burned bridges. These are false beliefs we concoct in our minds. It really all depends on how we say "no".
There are good ways and bad ways to say "no". The first thing you ought to do, if it's at all reasonable, is to ask the person to let you think about their request. You may not have the time or the wherewithal to handle what they've asked you to do because of some other responsibility or commitment you have. Ask them to give you a night to think on it. That way, it's a "maybe", not a "no", and they at least feel like you have considered it. If you realize that you really can't do it, you need to tell them "no" but also say something positive. The best way to say "no" is to a) say something positive and b) promise something else. For example, say, "I really wish I could do ___ for you." (That's positive). Then follow it up with, "Although I can't do ___, I can do ___."
This concept applies to all your relationships from work to your clubs and organizations. Simply say, "Even though I really wanted to find a way to make ___ happen, I couldn't. However, I can do ___.
Another tip: Give them a good reason why you can't do something, not a list of excuses. "I sprained my ankle, my kid's off from school at that time, etc." may all be legitimate reasons why you can't do something for someone, but you should only give one. You may think giving more excuses makes you look better, but in fact, it makes you look worse. If you start giving multiple excuses, it looks like you really don't want to do it. If you tell the other person in one sentence, "I'm sorry, I would really like to do ___ for you, but my mother and father are coming to town and I haven't seen them in quite a while," it seems more like you give a darn.
Sometimes you may not be the best person for the job. Tell them that. Say, "I'd really like to do that, but I don't think I'm the best person because I'm not good at 'X', 'Y' or 'Z'. But Bill or Mary is."
Somebody recently contacted me online at my Dr. Laura Designs store asking me if I could do a particular project for an event. I told them that I would look into it. I didn't want to say "yes" because I didn't know anything about how to do the particular craft, and I didn't want to promise anything I couldn't do. I did some research and realized that the learning curve for me to figure out how to do it would probably be a month, and the project was due in a week. So I responded back I would have loved to be able to do it but I couldn't because I didn't know how and couldn't figure it out in time for the event. I felt bad. I don't like to disappoint people and I really do like a challenge, but time constraints and my lack of expertise made it difficult for me to follow through.
Finally, if you don't want to help someone because you think they're using you or they're just a crummy person, you don't need to say so. Even though you may be thinking, "I hate your guts and I'd rather eat frogs than help you," that's not the kind of thing you should say to anybody unless you really want to get them out of your life for good. It's always nicer to tell a truth that isn't so ugly. Simply say, "I regret that I'm not able to do this for you. I hope you can find somebody else to help you," as opposed to, "Drop dead!" or, "Go to hell!"
Learning to say "no" is important because many of you let other people devour your lives out of a false sense of obligation. You end up having too much on your plate, which means you won't do any of it very well, and that's not morally right. Sometimes you have to disappoint people in order to maintain healthy follow-through on the obligations you already have.