Hang several signs around your work room that say "Just Say NO" and one on your activity calendar. It is easy to forget our resolve until we get in the middle of the situation.
I'm having a similar situation now with booking too many back to back teaching assignments where I have to prepare kits and not sure of the enrollment.'
I learned long ago in relation to another outside activity that I wouldn't do it if asked. I might do it if I vounteered because that was my decision. Same with a quilting donation for a local charity event. I will no longer be available when they call me. I might volunteer to do a piece but it has to be my decision and not pressured to do it. This latter was when I found out my work was being used as a door prize and not a raffle to raise money.
So do whatever you need to to remind you of your resolve. We tend to forget such.
I have resolved for 2012 Not to do any more round robins, block exchanges, fat quarter exchanges or jelly roll exchanges. I have two rr sitting on my work table now to finish up and am late, am late, am late getting them to the next point and a quilt to pack to send to a men's exhibit, etc etc etc.....Am diligently watching TV for a medication that solves this problem and then I will "ask my doctor if XXX is right for me"....