The only advice I can give from my experience doing digital art myself is you might want to take a look at some digital art tutorials for gimp and even some of the photoshop ones might help too. If you added some advanced effects to your work you might find it a lot easier to get good results..
One thing for example on an earlier picture i remember you saying it was too hard to do the skin tones when you already had detailed the hair... The trick there is to learn a bit and experiment with the "magic wand" and "selection tools". if you for example have the face not yet colored in but it's currently all one color or it has hard line borders of another color around it, you can use the magic want tool to select just the face. Once the face is selected, you can then paint freely and no matter how vigorously you scribble your painting it will only paint within the selected area. This gives you an advantage over traditional painting because you can paint long strokes to do highlighting and low lighting that normally would cause it to bleed out on to other stuff.
Most paint programs let you set the magic wand to grab a whole range of colors too. So you set the fuzziness of the magic wand to say 30 and when you use it, you will select anything within 30 shades of the color you clicked on. Then you hold down the shift key and click a second time somewhere else and can add anything within 30 shades of that area too to the selection.
Also re selections, for real detail you can also combine magic wand with the zoom feature and manual marque selecting (thats when you use the box or free hand select tool. If you weren't able to get everything you needed selected with magic wand you can zoom in hold shift again and use the marque to manually select pixel by pixel even if needed.
Lastly another advanced method is to use layering... Lets say you drew a very awesome looking figure but the figure is drawn on a plain white background and you want to now fill in a background. Well you use the same magic wand tool to help you here. You click on the plain white background with the fuzziness setting at 0 so you will only capture the pure white non painted on parts... Now you can paint freely without disturbing your figure. Also you could do the same thing select the background then goto the >> menu >> edit >> select >> invert selection Now instead of the back ground being selected only your figure is selected. This is usefull because you can then do edit cut... and then paste the figure into a completly different background scene by opening that document and doing edit >> paste.
BTW I was kinda typing my suggestions / advice based on you saying that you didn't know much about doing digital art, I hope I didn't tell you too many things you already knew...
Also I hope some of that made sense if not ask and ill make it more clear i might even be able to use some screen shots to show better what i meant.
Thanks for taking the time to type all this out, it is really good this advice - I will try some of these ideas out - !