I found this to be the most helpful guideline I have yet to find, and I figured other forgers would find it helpful as well. http://www.goodstuffmaynard.com/portfolio/independent/cdg/index.html
To anyone who makes or wants to make competitive maps (whether it's MLG or conventional), I would highly suggest reading this (At least the Map Design Theory Section). It is very simple to read and extremely helpful. It mainly approaches 1v1 designs, but most of it can be applied for 2v2 and even 4v4 maps. Hopefully this can get a sticky.
Yup. I remember reading this one a while back. Great guide - even though it's written for Quake, the core gameplay ideas still apply to Halo.