If most the grime is mud and such, try using your garden hose first. Remember to shut the vehicle down before washing. You don't need a lot of pressure to get the big stuff off. If you decide to use a pressure washer, consider using one that is 2000psi or below and try to avoid direct contact with electrical items or radiator fins.

After the mud is off, you can use several good products to get to the sludge. Personally, I like to use solvent. If yu don't have a solvent gun, then get a good spray bottle with an adjustable nozzle, You will be able to spray area liberally and let it soak in, then later spray it off with water.

If you going to compete with the pik that Chris sent, then you will need to plan a weekend of scrubbing. (I'm impressed Chris) You could also take it to a detailer, but plan on dropping a few bucks.