I recently donated to the American Heart Association. On their confirmation page right under the receipt information was an area asking me if I wanted to be added to their list. 
I thought it was a great way to ensure that donors have opted into receiving offers from them. They clearly outline to donors what they're going to get and they even tell you how often - monthly.
You might want to include an opt-in form on your own confirmation pages. Whether you're a non profit or you're selling a product, if your customers or donors just had a great experience with you, they'll more than likely opt in. By the way, needing a username and password is really only necessary if they create an "account" with you. Many times an email address, name and a few other pieces of information should suffice.
Any other ideas on growing your non-profit list?

I work with a local "Friends of the Library" non-profit group.
One sure way to get more subscribers I found was when we introduced a "thank you" page which reminds people they need to confirm their subscription (we use double opt in).
Before using the thank you page, our confirm rate was below 50%. After, it's almost a perfect 100%. This simple tweak more than doubled our subscriber rate.
Not all Email Service Providers have this feature, so chose wisely.
Posted by: Boris Mahovac | April 23, 2008 at 09:53 AM