I've found that I can get my in-house high-res illustration work finished faster and without a major headache (the little ones will always occur) by planning and working with certain points kept in mind:
1.) Figure the art will be used for a variety of projects...not just the one the original request was for (for example my worst case scenario -- it gets blown up to 10 feet high when the first request was for letter-size)
2.) Get the layout approved, but make sure there's enough stuff around it to allow for various formats (vertical, horizontal, round, diamond...whatever fits)
3.) Realize that other projects may take priority so it will have to be set aside at various times to work on other concepts
4.) Realize that it may get cancelled and never finished; or it may float back up after a year and need to be completed -- quickly
While I'm doing my painting in Photoshop (I very rarely work with Painter), I start with the high res sketch...usually scanned at about 450 dpi and the working file maybe about 11" x 14". From there I'll do a quick pass with the background and make up a palette for the major colors. Key elements I'll do what I call a middle color pass, under the lineart, usually just a solid color to separate the elements into individual layers. Gives me a chance to see if the color scheme is working or not.
Since the file size gets pretty big with too many layers (125MG/900MG as shown in the Photoshop panel at lower left the way I have mine set up), I generally do the background details first with various layers, and then merge them before I work on the main characters. I want to keep the 2nd number under 800 MG as that in my experience, keeps Photoshop and my machine happy. In any case, I definitely save iterations of my projects (version01, v02, v03, etc.). That way if things change, or the file goes wonky, the art gets used in another project, or whatever...I still have a file somewhere with enough layers to more rapidly do the updates requested...or at least have enough art that I don't need to start from scratch.
And the images posted today are from one such project that isn't finished. Another work-in-progress, in fact. Ha.
The first image with the red silhouettes shows a part of the initial sketch with flat color to break up the art for the detail color passes. The second image shows updates to the character lineart and positioning (I wasn't happy with the original sketch...but not because I didn't do it...) and some of the color choices I was making for the character on the right; and that I'm close to final on the background art. After I'm satisfied with the middle value coloring I've done under the drawing, I'll do an "over-painting" layer. Basically, a layer over the lineart with the detail painted over the lineart and the middle value coloring. My final steps will be to add final highlights and shadows over the entire piece once the character detailing is complete. Basically, start broad and work over the entire canvas; then spiral in for the detailing; then back out again for final tweaks. Makes sense to me, anyways.
The close-up view of the deer at the top shows some of the overpainting in progress, the lineart, as well as the middle value coloration under the sketch. BTW I cropped all of the images in to focus on the characters...there's alot of elements to the sides that aren't included in the posted images shown here. You can also see that the illustration is in no way complete...particularly since all the detail at the moment is in the head of the animal; and the background trees are still floating above the snow... Maybe one day I'll get to finish it. :)