You are approaching tens of thousands of people, what, four times a year? You can't develop a personal relationship with all of them. You can't afford to send them direct mail pieces. You can afford, through the generous efforts (and mental tenacity) of those willing to do it, carpet bombing. It is an effective way to get the point across.
I would suggest that "the committee" or whoever the FR group is who looks at these things, take a look at those who give regularly and target those individuals for a more personal commitment to FR, both in time and money. Some will be able to give both; some one or the other. All will appreciate being asked and will more than likely be there for you in the future in ways you could have never dreamed.