There's a lot of information about DMing in L4W buried in various threads. This is a list of some of them.
- Hoofchew: Behind the scenes covaithe documents how he built, proposed, and ran a simple short adventure.
- Possible New DM Questions Some questions and answers. There's an excellent post by H.M.Gimlord on mapping resources.
- Another thread on mapping software.
- Skill challenges
- PbP DMing tips, a grab bag of useful tips.
- The Upcoming Adventures thread has lots of DM-related discussion. Lots of noise, too, but it's a useful thread to watch if you're a DM.