Note to self; Learn to say NO

Am I the only one who has let myself become a "pee-on"? I've discovered today that when it comes to people "needing help" I can't say no! It doesn't seem to matter if whatever it is they are asking from me, is something I can actually do, I say yes anyway! Whether it's dinner for this person or dessert for some other function, I just can't seem to help myself! I swear I must wear an invisible sign that says "Ask me because I will say yes".
Okay, so I know I should have a better attitude about this as it falls into that "serving others" category. But seriously, where is the line? When is it ok to say "no, I just can't", and when is saying that just being self-centered?
So I just needed to vent a bit and i'm good now! :)