This tip is very, very simple and yet very often overlooked by most Jewish organizations!
If someone takes the time and effort to donate to your organization either online, by telephone or by mail, take one minute to write them a personal (notice how I said personal and not form letter) email or letter to thank them.
A personal letter or email will go a long way in acknowledging a donation and retaining that donor.
The risk of cost and time is minimal and the rewards are potentially great!
Yoav