I found this thread over at the MLGpro forums (I even stole the thread name) and since a lot of this community doesn't regularly lurk there like I do, I thought it would be beneficial to post the link for anyone interested in it's epic read.
Keep in mind that this is written for Quake 3, but a lot of what's in this (really, really long) guide can easily translate to Halo 3. Hope it helps.
EDIT: Many thanks to Novak for the updated link. The previous one no longer works.
Competitive Level Design Guide