Fans have been selling fake lilacs on towels, pins and T-shirts as part of the "Match it for Pratchett" effort to match his half a million pounds donation to Alzheimer's Research.
Here's a Squidoo page one of my fellow Squids made explaining Wear the Lilac Day.
In short, you can buy lilac pins or goodies from fans who are donating the proceeds to Alzheimer's research (that Squidoo lens has a few), or go to the Match it for Pratchett website and donate directly with Paypal. And on May 25, wear a sprig of lilac!