Declining a donation request is a sensitive and awkward process. Manage it carefully and try not to hurt the feelings of the person asking for the donation. You have your reasons for denying a donation request: Perhaps you don't have the money or just don't believe in the cause. Either way, it should be done with sensitivity.
Create a formal notarised letter from your home or office. Use the same professional standards that would be used in any business letter: Print your contact information at the top left of the letter with the non-profit organisation's contact information beneath it.
Compliment the organisation's efforts at the beginning of the note. Outline the nature of the work the group does and why you think it is valuable.
Politely decline the request in the second paragraph of the note. Depending on the reason, it may be wise not to share too many details about why you are declining. If you have had financial trouble that has become public knowledge, you may be able to point to this as a reason for your inability to donate.
Extend an offer to contact you again in the future in the event you are able to donate later. Give a specific time frame for the group to reach out to you again. Compliment the organisation again and offer your best wishes to the group. Close with a respectful salutation.