Remember that you can be prosecuted for leaving off this information. Sometimes, you can include information on the card which won't fit neatly into the body of the letter. Favorable quotes from the media or from well-known people are examples. Also, request their phone number. Always, always, always, include one.
Nothing deters a potential donor like the lack of a return envelope and sufficient postage.
Buy a Business Reply Permit from the post office. This sounds obvious, but check it anyway. Each BRE will cost you a fee which is nothing compared to what is inside. Teasers Teasers are the words written on the outside of the envelope to provoke initial interest. The most successful teaser is a message which suggests that there is a positive benefit to be gained by opening the envelope.