-Fog your motor (search for proper procedure)
-Drain gas tank or fill to top with stabilized fuel (stabil for ethanol)
-If filling tank, run the ski for a few minutes to get the stabilized fuel in the carbs
-grease all fittings
-coat all metal parts in fluid film
-for extreme climates, flush cooling system with antifreeze
That's the basics of it from what I recall.... Down in Texas we don't really ever winterize
PWC today has plenty of topics on winterizing as well