"No act of kindness, no matter how small, is ever wasted."
~Aesop

I like this one because some people, whenever they are asked to do something, just feels like it's not a big difference, and they just do it because they want to. But even if all they did was go get this or that, or lend the other person something, that's still very much appreciated. So you shouldn't think that a tiny favor is of little or no importance, because it is.

"It is better to remain silent and be thought a fool, than to speak and remove all doubt."
~Abraham Lincoln

I really like this one, because I have a tendency to not think about what I say and just say it. It means that you should think twice before saying what you want. If you say nothing, people will only think that you're a fool, they don't know it. However, if you do choose to speak out, and you say something stupid, they'll lose all doubt that you aren't a fool. Everyone's a little rash, so this should be something to keep in mind.