A positive, inspirational tone in your letter is better than the doom and gloom approach.  Donors want to support successful organizations that have big vision, plans for achieving great things and solving big problems.  Gone are the days of sending out the “if we don’t get your gift this month, we’ll be forced to shut down XYZ program.”  Donors are much more sophisticated and have access to reams of data about nonprofits today.  They have many more options than just your organization.  So inspire them – show them how they can change the world through your programs.