I've printed this list of "How to Read (ie what order) Terry Pratchett's Discworld Books" although I reversed the sections. I put the "publication order" section at the end, and prefer to read them by the second ordering.
http://www.poslarchive.com/math/misc/discworld.html
I've tried to read them in publication order, and I always get hung up on the early ones, which I can no longer make it through (too trite). So I was for years stuck on book 3 or so, and never made it into the really good books of the Watch series. Big Mistake!!! Now that I've started in more or less the 2nd order .... I am proselytizing Terry Pratchett books to anyone who will listen to me!