Well, assuming you have the brazeons and mount points for a rack, it's not difficult. If you don't, it's still not hard.
With mount points: The only concern is heel clearance, a mountain bike has short chainstays, so you'll need a rack with a lot of setback for heel clearance.
No mounting points: P Clips and the seat post tensioning bolt are your friend.
http://www.bikeforums.net/archive/in.../t-174528.html
Another option is a combined Trunk?pannier bag (Trunk bag with zip out panniers), and Beam Rack that mounts on the seat post.
http://www.cyclexpress.co.uk/.%2fpro...m_Rack_EX.aspx