Check out the updated
Discworld Reading Order Guide 2.0, which, like version 1.5, shows the different sub-series within the entire Discworld series, and shows which books are directly related to which (and also indirectly, usually through passing references and cameo appearances).
I mentioned in another thread somewhere that I like the Rincewind and Death novels the best. The Colour of Magic and The Light Fantastic should be read back-to-back. One of these years, I'll finish reading the Watch novels.