First things first
The very first thing you should be doing is having a read of your PWC's manual. All ski's are different and you should be aware of any subtle differences in them, to what we are showing here on the Sea-Doo models we have and use.
You want to wash the ski in a spot that's flat and preferably in the shade. If there is any incline, have it parked with the nose of the ski facing the higher area. Also, if you don't want to become a viral internet sensation, get some good wheel chocks on the trailer so it doesn't roll away on you - these can be found in many automotive stores.
After each ride you should flush the engine, especially if you've been in salt water - check your ski's guide on how to do this. In the case of the Sea-Doo here, you can simply connect up a hose (don't turn it on just yet), then run the ski on idle and then turn the hose on. It's a good idea to have a hose (as pictured) with a flow valve on the end to make this easier. Leave it running for what the manufacturer recommends, which can be from 30 seconds to a minute. This will flush out shells, seaweed, sand, ski rope threads as well as importantly help prevent any dreaded corrosion before turning off, unclipping the hose and giving the engine a couple of few small revs to clear any remaining water out.