Telling people "no" has always been difficult to me. Not to burn-out and to actually enjoy the things I do, I've started to learn it. Here's how
1. Don't answer right away
Ask for time to think. I've noticed that it's much easier to say no (if you need to) when you have time to think things through.
2. A person who is tired and not passioned can do more harm than good
You might ruin things if you say yes despite being tired, stressed out and not passionate about the cause.
3. It's not personal
Stop thinking that you are insulting someone by saying no. You might even do him/her a favor: see number 2!
4. Do something well, not many things poorly
Always remember to ask yourself: "If I say yes, am I able to do it well?"
Learn the art of saying no