Ask your volunteers to plan a gift!
This is really very simple.  More and more seniors are volunteering these days.  In fact, Philanthropy News Digest found the following:

• Seniors are now volunteering at record levels
• One in three volunteers is 55+

Combine this with the well-known fact that people like to leave bequests to organizations with which they feel they have a relationship and VOILA!…  you’re in bequest central.
The best part…  asking your volunteers to consider a gift in their estate plan is easy and costs virtually nothing.