I haven't read any of the discworld books, but I had a similar problem with Frank Herbert's Dune series and Isaac Asimov's Foundation and I Robot series, because they weren't published in the story's chronological order, which was the way I wanted to read them.

I solved the problem by searching for something like "Dune books in chronological order".

@Mslion, the Wikipedia article on discworld has pretty good information on reading order if you say wanted to read all the rincewind books, or all the witches books in one go.
That being said, I personally always think it's better to read discworld at least in publication order because that way you get to see the eclectic buildup of the discworld povs, especially in books that feature miner appearences by main characters from other parts of the series.

For example, Guards Guards comes rather early in the sequence, (number 8 I believe), and yet there are lots of other books from then on that members of the watch appear or are mentioned. The same goes for Wiliam Deword and the staff of the Anche morpork times, sinse while they appear first and are main characters in just one novel, "the truth!" they appear from then on in various guises reporting the news, and obviously it's much nicer when seeing those future appearences to know where they've come from.
Raising steam, which is the 40th  and last (up until recently), novel Terry Pratchett ever wrote, is almost a who's who of discworld, and though it's mainly about Moist Von Lipwig,   most other major discworld character's, even people like Rincewind make brief appearences in the book as well.
That's one of the nicest things about Discworld, it truly is "A world!" sometimes the stories of the characters you follow are just one book, sometimes they're many, sometimes they're introduced once for one book but appear again later.