There are a number of threads on lowering.
A few ideas are:
Drilling a new mount hole on the swing-arm to move the bottom of the shock further back.
Changing the pre-load on the stock shock by moving the spring retaining clip (undocumented mod not found in the service manuals).
Buying/installing a different shock (shorter or lighter spring).
And finally, adjusting the front triple-trees lower on the fork tubes to match the drop in the rear.
Just remember that you will be reducing your available suspension travel.