when you mean “no”, it’s not the end of the world. go back and find that person and explain you made a mistake and that you actually can’t help on this occasion. i know for me, sometimes i find it difficult to say “no” on the spot, but another thing i have found useful, is if you hear a request coming your way, give yourself some time to consider it. let the person know you are considering it, but need to check your schedule, and that you will get back to them. this gives you the time to see if what is being asked of you, works for you too.
life 