If you are looking to donate time, look around your community and you will see an activity/fundraising/whatever in which you will see a person (or people) without whom it just wouldn't happen.
Offer to help them
. In my (limited) experience, the burden of that sort of thing always falls on a few people with a truely charitable heart, and the more help they can get the better for them. Basically so they don't burn out.
I do Church youth group/young adult stuff, but realise the Church bit isn't for everyone as a charitable outlet. It is a pretty sweet deal for me - I am in a group of 6 who organises stuff, so we spread the work around a bit.
I guess what I'm saying is that if you are going to give your time, make it in a sustainable way. I've seen too many people burn out through taking on too much (the pinch comes when work etc gets busy too).