Justin "Scorpion Zero" Jones
I ran the Warrior Dash...which is a much more tame version of Tough Mudder. I was planning on Tough Mudder but I've since been busy with other things than training.
Training-wise, I created my own training program. It mixed everything from Insanity/P90X to speed track workouts, to cross-fit, and pretty much the hardest workouts I could come up with. I've got it all laid out if you're interested. About 30 different workouts. I focused on speed and overall body strength and much less on super long runs and pure weight-lifting. I turned into a fast, lean, and completely toned beast. It was awesome.
As far as advice...don't make the same mistake I did and take it too seriously. These extreme obstacle runs are supposed to be fun. I hated it when I ran my race because I won my heat by about 2 minutes so I ran by myself the whole time. The second time through that day I went half as fast with some friends and had an absolute blast. Make sure to bring 2 pairs of shoes, towels, lots of water, and sun screen. The obstacles in Tough Mudder are more serious, but again, it's an event, not a race, so have fun with them.