This article is from the Pratchett Newsgroups FAQ, by Orin Thomas email@example.com with numerous contributions by others.
He'll post when he's going walkabout, so just bide your time and see.
The alt.fan.pratchett www page usually has some information in this area
if you missed it. Generally he will sign three novels per person, and,
if there is time, get on to the others.
The Stamp which reads EX LIBRIS MORTIS - HIC EST VITA VESTER
FROM THE LIBRARY OF DEATH
THIS IS YOUR LIFE (S Briggs Translation)
There is a Web Page on the Lspace Web with tour details.
He generally goes on tours after a book is just published.