basic things; make sure the fuse box is closed and sealed, some people cover the distributer, if you have an itake with the filter exposed in the engine bay you will want to have that covered. I can't think of anything else at the moment.

some people just go to a DIY (Do It Yourself) pressure wash type place, pre-wet the engine bay - don't soak it, spray some Simple Green (stuff works good for me) over the areas you want to clean up, let it sit for a moment then use the pressure washer to wash it off - repeat if not satisfied the 1st time around.