Years ago, Orin told me to put two area washers (the standard kind, not the ones Scott Krueger mentioned yesterday) under the TOP two engine mounts, thus "building in" some sag.  Then, when the engine really sags due to long term deformation of the engine mounts (rumored to be within the first 50 hours or so, then it stays constant) you remove the area washers.

He recommended this approach ("pre-sag") rather than adding the washers to the bottom mounts "post-sag" since he felt it was easier to add/subtract washers from the top rather than the bottom.  Otherwise, there's no real difference.

- Rob Wolf