Do you have a pressure washer?? I haven't recommended any products as there are so many out there with lovers and haters of all.
If so, opinion is divided here on wet or dry application of snow foam. This what I do with great results on a glacier white car.
1) Jet wash it first with water to remove the top film or the really ground in areas.
1a) Spray Tar-X or Korrosol onto panels where you can see 'spotting', DON't let this dry, jet off once you see it re-acting.
2) Snow foam it, leave it 5mins or so, try to not let it dry.
3) Jet wash it all over. Repeat if it's really, really dirty.
4) Wash with a good quality mitt, lambswool etc.. with the 2 bucket method, invest in a grit guard for one bucket(the rinse one), or both to reduce rubbing the grit back in
5) Jet wash to rinse off
6) Dry with a large drying towel, Kent or Wooly mammoth. (before I dry off each panel I spray a diluted mix of demon shine on each panel that helps disperse the water to aid drying, also leaves a decent streak free shine if you can't be bothered to wax afterwards)
7) Polish, wax and seal
Without polish or waxing, takes me about 1hr to get our current G.White Megane RS looking superb.
If you can, clay it at least twice a year then treat the paintwork to a quality wax and sealant. This'll help every time you wash it as more layers of wax and polish will help prevent road grime getting a good hold of your paintwork. The beading when it's wet is quite pleasing to see for some strange reason also. Sad I know but getting it / keeping it clean can get quite obsessive.