You could go out and buy a couple of tins of gunk or similar, but it's expensive stuff and petrol does just as good a job. Take a jerry can of petrol, a paintbrush and a hard bristled cleaning brush (a dish washing brush and a toothbrush were my weapons of choice

). First clean the loose rubbish off with the hose and brush, then let it dry and paint the petrol on. Leave it five minutes to soak in, scrub it with the hard brush and then remove it with a hose and the cleaning brush. Repeat a couple of times as necessary and it'll get the grease off.. might then be worth attacking with a wire brush, then painting it while it's out. There are also some vinyl coating products that you can paint on the engine bay to keep the grease off and make it easier to clean, although I don't know if that'd stand up to the heat on the engine itself.
One thing to remember, first time you take it out on a dirty motorway it'll look revolting again, so you'll have to keep cleaning everything every couple of weeks to keep it looking half reasonable.