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 contrast, set it to "multiply" mode, put it on the top layer, and lock it (important! You don't want to end up colouring on it by mistake) the lines and any shading from your sketch will be visible throughout your work, whatever you do underneath.
Then you make one or more layers below that, name them "flat colours" or something like that, and using the selection tools, like magic wand or polygon selection, you mark off colour areas of your drawing, and using the fill tool or a large brush, colour each area in the midtone for that area. If you then do all your shading, highlighting, and textures on yet another layer, you can always go back to that "flats" layer to select each area, no matter how messy or fully rendered the painting you do over it!
The second technique is sort of the opposite approach, and my girlfriend prefers my work when it tends more to this extreme, but the truth is that they can work together and compliment each other very well, and the best digital painters, like Ukitakumuki, Shimmering Sword, Rahll, 343GuiltySpark, etc probably use both. I'm talking about slapping the virtual paint and textures all over everything like a digital Kandinsky! This is unquestionably the freer and more dynamic approach, and leads to the happy accidents that can really bring life to a work. The genius is in combining the two. I know I'm not there yet, and as you say yourself, practice is the key, but basically, don't be afraid to experiment, in digital media, there's always the undo key!
Oh and about that locked sketch layer on top, once the painting underneath gets to a certain level, you can unlock it and start to play with it. Change the opacity and blending modes, selectively erase parts of it, you may find that the suggestion of pencil texture adds to work in places, while in others clean colours are better.
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