The masters at this are the plastic model guys. Get yourself some plastic model paint. Mix some brown, black, grey, essentially what you need to make it look like a dirty mess. Then thin it about 80% i.e. really runny - only 20% paint - which is generally known as a wash. Then spray it on, forget about worrying about runs etc. Then, take a cloth and wipe it off. It will collect in the panel lines and around any lumps and bumps, just like real dirt. Then, on panels that are often removed for maintenance, mist on a very subtle fog of the "dirt" around the edges of those panels, from far back, say 6" either side of the line in full scale terms. This really brings the finish alive. Look at pictures of military jets in the field and you will see where they are dirty and how little subtle shading of dirt, dust, exhaust etc makes it look much more 3D.

There are lots of articles on this in plastic model mags and books. These guys are the real masters of the art.