One universal tip, remember to always thin your paint down with water. It goes on smoother, and dries very smooth too. It may take a few more coats, but it's worth it.
For the "machinery metal", you know, the metal that gets all the heavy-duty work, I paint a basecoat of black, and then I drybrush Boltgun Metal over it. Very nice worn metal look, trust me. And then maybe paint the hydraulic pipes and other things like that full Boltgun Metal on top of the drybrushing, so as to add a bit more variety.