Include a story about one particular student, one particular endangered species, one person who conquered a disease -- something real and relatable. Use that person's name in the story. The girl in the photo is named Chaya. Now that you know her name, you feel closer to her story, right?
Never ever begin your letter with "I" or "We." Always make it about the donor.
Make it financially tangible. What will $100 -- or $100,000 do -- to change the outcome for the better?
Include a time perspective. What will happen in a year, or 10 years, if the situation stays the same? Give your ask some urgency (but avoid desperation).
Let the donor know that there are other donors to the same cause. That as a donor they are part of a team who all believe the same change is important and necessary.
Be grateful, be generous of spirit, be respectful. And keep your letter SHORT.