Tom gives two good ideas for how to achieve this. These include specially acknowledging donors who've been with your organization a certain number of years as well as acknowledging ones who've given certain amounts.
What I like about his ideas is that they're original (not that I know what every organization out there is doing, but I personally haven't seen this twist yet), they're easy, if you're already keeping decent records, and they're the very opposite of what feels like the more common if perhaps unconscious approach: that is, the one where donors get endless appeal letters in which the organizations practically hide the ball, not telling you how much you've donated of late, as if afraid that once you notice, you'll turn off the tap and say, "Forget it, I'm not giving a thing to this next appeal!"
Why not celebrate how much a donor has given, and foster their pride around that -- especially if you can send them a tailored letter explaining what has been achieved during their three years of membership or using their total funds? I'll be curious to see the responses to Tom's post . . . .