Signing the Letter
The signer -- and there should only be one! -- should always be the person with the most name recognition on the particular issue you're addressing in the mailing.
So why is this very simple thing so complicated to put into practice?
The reasons are endless. A nonprofit might have two figureheads (a President and an Executive Director, say) who both feel they should be the ones signing letters to donors. Or it could have one leader who is very well-known for one specific issue -- even though the organization is working on several issues -- who insists on signing everything. A nonprofit might have oodles of celebrity support, but be afraid to ask for celebrity signers. And on and on.
Organizations should consider each letter they send out as a new opportunity to bond with their donors. Which means they should think carefully about what issue will do that and who the best person in the organization is to address that issue.
In an organization with a particularly strong or charismatic leader, it might be that leader every time. In an organization that has two distinct audiences -- say an activist human rights group with a strong education program -- there may be one leader who is perfect for addressing the activists on the list, and one for the education supporters.
One environmental group I work for has a celebrity -- in this case, an actor well-known for his environmental advocacy -- sign a letter for them a few times a year, while the executive director signs everything else.
Above all, your letter should always have only one signer. Remember, fundraising letters are personal letters from your organization to your donor. They should speak directly to that ONE donor, person-to-person. And they can't do that if they're signed by two people.