Start by using the Top camera for accuracy.  Select the body.

Open the Joint Editor (alt-W-J) and then locate the green cross.
Grab the green cross with the mouse (cursor will turn to a black bulls-eye thing
when it's grabbed) and move it to the "bottom center" of your model. 

You may have to zoom in a couple of times to get an accurate center,
if the original center is way out of bounds.  After zooming, be sure the
Body is selected again.

The X and Z will generally not end up at zero, but you can manually set the Y to zero.

Resave the model.

(Note: the Align button sounds like it should be useful here, but it's really
meant for lining up the start and end of a bone to point the same way.)

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