Depending on what type of charity it is - one of the best ways to get tons of traffic to it is to contact your local newspapers and media outlets to see if they'd be interested in doing a quick story on it. I know, it sounds out of reach, but honestly, it's not. Get a good press release written up for it, and send it all over the place.
Networking For Charity with the people who devote time for social cause would help you, I think you should look for social networking and fund raising sites which support people engaged in social activity.
The thing is, to get any news coverage for it, it would need to get big enough. But to get big enough, I'd need lots of money which I don't have.
Word of mouth, build a reputation and tell ppl about it. I prob wouldn't do ANY internet advertisng, except MAYBE some adwords.