parker17- In a nutshell my photoshop work consists of soft light and multiple layers over the model.
I use the Dodge and Burn tools in Midtones Mode to bring out the highlights and to deepen shadows. If i need to add any extra bits to the design I simply paint on new design elements.
I use photo textures of metal and dirt and any other texture that i feel would work for the design on soft light layers to give the work a more realistic feel. And I play around with the opacity of these layers to see what works best.Photo Textures and soft light layers are amazing tools that add to the design without altering the model.
As a finish I create two layers of Soft Light called cool and warm.I use a cool color in the shadows and a warm color in the highlighted areas to bring out the design even more.
That's my basic process. Let me know if you need any more info. I personally dont' use masks, but they are a great tool.