Clone
Clone your drive in case there is a problem with the install, you can revert back to your clone.

Defrag
Defrag your drive to greatly improve performance.

Driveslim
Use DriveSlim to strip out all the extra languages supported in Mountain Lion and remove all the large sample videos, freeing up hard drive space.

Repartition
Repartition your hard drive to keep a Lion partition for any apps you have that aren't Mountain Lion ready.