Off the top of a Google search:
http://doom.wikia.com/wiki/Tips_for_creating_good_WADsThere's some pretty interesting stuff there. It starts with the obvious but there's some neat stuff later on (the "Always changing floor height when I wanted to change floor textures" one is not that obvious).
I wonder how they compare against Sonic Robo Blast 2, which has some stuff that goes against some of the tips there (e.g. the flower on grass floor texture is meant to be used at the same height as the normal grass texture), and some unique to it (e.g. the grass border middle texture to help distinguish grass boundaries, Doom has nothing like that to worry about).