Back when it was dry and reasonably warm, I gave it a good wash, and clay-barred it, using Meguairs clay bar and lube fluid combo from Halfords. Once done, a coating of Turtle Wax Hybrid Sealant Hydrophobic Carnauba Spray Wax.
That left the paintwork feeling smooth as silk, and caused rainwater to fall right off. The Turtle Wax coating has lasted for ages. Absolutely ages.
I used to use Autoglym body shampoo and wheel cleaner, but I've found Simoniz products in Tesco that do the job just as well at a fraction of the price.
I've used snow foam (again Autoglym) with a power washer, but to be honest, I didn't really get the impression that it did very much. Felt that I still needed to wash it, and that it hadn't taken much, if any, of the surface dirt away with it.