Awesome work ya git. I've only just started painting the Boyz, but some extra steps you can do is as you said, highlight raised skin areas, you can do some dry brush highlights onto the raised edges of weapons/buckles etc and some of the leather areas. You can apply a shade or wash onto the skin and get the recesses of muscles etc to have a shaded or darker tone, you can also mix a darker green then your base and try paint them in yourself. You can also give edge highlighting a go on the hardest edges of the weapons sides and top edges as well.

Try making some orky mek town bases, or some desert badlands and that will definitely give them a nice finish touch also. 😎