Ground-Clamping Algorithm

Hey everyone I''m working on a terrain engine right now, based on octrees and the collision detection code found at www.gametutorials.com... I just realized though, that the player is on the ground 99% of the time (except when jumping and falling off cliffs). So... I''ve been trying to come up with an algorithm that clamps them to the ground at all times, instead of calculating whether they''ve collided with the ground, but I can''t think of any good ones... The ones I''m considering are all so sloppy and unreliable... Any suggestions?