I like to use the blending modes like colour and overlay just to get a basic colour set and to help get the general feel of the colour values. I then flatten the image and work on it as a normal layer, improving the highlights and shadows as I go. That way I can avoid the irritating and inexplicable way the colours and values change when I try to add different hues to it!
There's a really good workshop in IFX issue 92 about portrait painting in Painter. The principles work just as well in Photoshop and they use minimal layers, which I have found to be a bonus. If you have the issue then check it out. If not, like I said before, just use the blending modes to start the colouring process and then simplify to just one layer and paint as normal. That's what I have found works anyway!!
It's not about volume, it's about value!