Goto to the motion panel, open the tag labels, create a new label, select the center of mass of the object's bones (if its a biped, select the diamond thing, if a bone search for a point, if there is none, assign a point to the bones)
For each key of the center of mass moving in the apropiate axis (say, he's walking alogn the Y axis), set it back to the value of the first one (first keys is at y=10, so al keys are set at y=10).
As far as i know this works ok, thought sometimes you might need to retouch some stuff.
Further on my stuff at...