Learning to Say No

No, not me... I apparently haven't figured it out. I'm a real sucker when someone says, "Can you...". Logically my mind is screaming, "NO! NO! NOOOOOO!!!" when instead what really comes out of my mouth is, "Okay, sure."
Let me show you a couple of examples.
My friend is the PTA President for Ally's school, she is good at it, and I totally said I would help out... because I really do want to help. Well, a month or so ago she asked me what I knew about Reflections. "Nothing... I only did it a couple of times when I was in elementary and my teacher made the whole class enter." She asked if I would mind helping. "Sure," I said, "I can help with some posters or something." She smiled and thanked me...
Well a few days later she said she had a big favor to ask me. I was thinking she wanted me to watch her kids or something... then she nails me with, "Could you possibly be in charge of Reflections? I can't find anyone to do it..." So, I am pasting a fake smile on my face and thinking, "NO FREAKING WAY! I know nothing about it, I have a hard time even volunteering in Ally's class, because of Devin... I just don't have the time, I don't have babysitters for the meetings..." So, what comes out of my mouth? "Um sure... I can try."
It can't be that tough, right? Seriously this is going to be so flippin' hard. I have a committee to please of 5 women, 2 assemblies to plan, judges to get, posters and fliers, color coding of age groups, a presentation to give to every classroom... treats for all the participants... etc., etc. Oh gosh, am I in WAY over my head on this one. I love to help out, but I am SO not into being in charge or heading something up.
So, here is my second example that happened today. I get a phone call, but since I don't recognize the number I ignore it. (Heads up if you ever call me from a number I don't recognize, I won't answer). I listen to the voicemail and it is from Ally's school. Apparently Ally's teacher doesn't have a room mother and they really, really need someone, and since I signed up to volunteer at all the parties would I mind terribly if I just take over the planning of all of them? So here is my thought process this time, "I can just ignore it, pretend I never got the message. I can still volunteer at all the parties and help out in the class room... I REALLY don't have time for this one when I am already in over my head with Reflections..." So what do I do? I call her back and say I'll do it.
So, here I am in charge of two huge things that I had no intention of doing. And why? Because I just can't tell someone "no" when they need help. Can I do it all plus weekly play group and library class for Devin, piles of homework with Ally, plus times table and cursive and Acheivement Days and Enrichment Committee and the treats I'm supposed to bake for Alex's work meeting tomorrow... running www.spartanonlineshopping.com... not to mention just all the "normal" housework and laundry and meals...
Please, for my sanity, don't volunteer me to head up any committee's... because I just don't know how to say no!