There is no set rule or magic bullet format. I've seen 6" x 9"s work in one place and fail in another. I've seen the same components in a #10 work in the Fall and get beat in the Spring in a 6" x 9". Testing is the only thing that is tried and true.
Are there general principles when applying this to non-profit fundraising?
Fultz: Relevance is important. If you are mailing to previous donors, you need to make sure you treat them as if you know who they are. Also, given the fact the address labels are still a strong pulling element for nonprofits, there still is a little 'What's in it for me?'
We recently developed a folder style piece that contained a letter and order form with an integrated label that sits inside a mini-personalized folder. This creates a kit type of package that we took the concept to the maximum calling it the "Gift Giving Kit."
In terms of creative ideas I've seen for the non-profit world, in most cases I've been disappointed. Not so much from a production standpoint as I understand the pieces can't look expensive, but more from the conceptual approaches. It may be time for the non-profits to swing for the fences a little with concepts that are more unique.