Check out these guys. They make bike fenders for just about every application. Ive been running the GRAND D.A.D and GRAND M.O.M fenders on my all mountain rig when it gets sloppy and they work great. Keeps the muck off me and they stay in place extremely well even when hammering through rock gardens etc. They do give the bike a bit of a moto look when installed so saying "braap" is mandatory.

http://www.sks-germany.com/?l=en&a=products&r=mudguards