I usually do this:
-basecoat with a dark grey ( say Charadon Granite, but it can be any grey, it just has to be veeery dark)
-heavy drybrush with less dark grey (say adeptus battlegrey, but any equivalent is ok)
-light (and i repeat, light!) drybrush with codex grey
-thin highlighting/lining along the edges and raised areas with a mix 50:50 codex grey/ceramite white
hope it helps, cheers
-nasty and reckless washing in nuln oil.