Good advice here..
http://www.polishedbliss.com/
Spend a good few hours the first time getting it really clean (just go easy with water pressure - more of a sprinkle really) - use the right products to protect it and from then on it's an absolute doddle to keep it looking new, just 20mins once a month or so.
Autoglym Rubber and Vinyl Care is great for under the bonnet - spray on when wet and just let it dry.
Same with the underside, give it a really good scrub (those jetwashes with the foam brush are good for this if need be), apply protection to the wheel arch liners and all plastics - I actually take them off once a year to really give a good clean and waxoil behind etc.
Autoglym Bumper Care applied with a paintbrush is good for the underbody plastics - nice, black, new looking wheel arch liners always set the alloys (and the bodywork) off really nicely.
A bit of work protecting these parts makes all the difference, dirt, salt etc in winter just hoses off and I'm constantly surprised how good (clean and rust-free) the underside, wheel arches etc of my car look compared to others when I take it to the dealers.
Cheers, Geoff.