I wrote this on a new deviant's journal, thought it might be worth sharing more widely.
I dunno that I'm really good enough to be offering advice re. Photoshop yet, but I'll go ahead and do so anyway!
There are a couple of contrasting schools of thought that have influenced me, and I find both useful in different ways. Firstly, the methods of block colouring and using "flats" that you see a lot in Manga and other comic styles can be VERY helpful, even if you're going for a "painterly" result. The important thing is layers, and making full use of them. You start with your sketch, whether digital lines or a scan, you clean it up, make it high