Hi lamamini,
When I picked up my S last year the first thing I did was spray the underneath with Dinitrol underbody wax. Took about 4 large aerosol cans to do the job and takes no time.
It gives a nice brown metallic smooth finish to the underneath, and did all the suspension arms etc too. I have reapplied the wax again last weekend ready for the coming winter. I only ever wash the underneath of the car with a standard garden hose which gives enough water pressure for me. I'm not really a big fan of pressure washers after watching my neighbour one Sat afternoon strip all paint of the rear bumper of her Fiat Punto.
I bought the Dinitrol wax from
www.frost.co.uk, about a £8 a can. Worth it say to put a barrier between the underbody and the salt etc.
Hope that helps.