We just created a program for a client to take advantage of that. On the heels of any purchase, his Top and Second Tier customers receive an offer by mail. Conventional wisdom might tell you that we're mailing too soon, but experience shows otherwise. The mailing brings back 42% of Top Tier customers within a month, and 28% of Second Tier customers.
Whenever a customer makes a significant purchase, send an email or, better yet, a snail mail that (1) thanks the customer and (2) suggests an additional purchase. If you're a fundraiser, bring up a need and suggest an additional donation.