Have you considered some sort of rewards program where everyone gets back 120% of their donations when five new members donate?
I'm personally not huge on giving coin back. The MS Society does have a fundraising prize program, but I'd rather see the money go to disease research rather than a free coffee mug.
However, I'd be happy to send an MS T-Shirt or a Zazzle gift certificate to purchase your own (for those who dont like to give out their address) to the top donor if the total donations collected exceeds 0.1 btc.
March is MS awareness month so I thought it was a good time to get moving on the donation collections. As of the time of this post I've been sent 0.001982 btc in donations.
I know it must be hard to donate without proof that the coins will actually go towards the cause - and I'm not sure how to prove that they will. If anyone has an idea, please post.