I use Paint Shop Pro 8 (PSP), so this tutorial is sure to work in it. Other versions and Photoshop steps may vary.

What am I talking about? I'm sure you've seen artists post their digital art. Many say they scanned in a sketch and then colored it digitally. But did they have to "re-draw" it on the computer??
Not necessarily.

This is what I do.

Step 1: Pencil sketch some art (or however you usually work - or it can be an image from anything, really, that you want to makeover digitally but don't want to lose the images essential "lines").
Scan it in or get it on your PC somehow.

Step 2: Open PSP. Then open your art you just scanned in.
Now, if you started coloring on this puppy now, as is, it'd be solid color plowing right over your pretty little outline or sketch. That's not cool.

Step 3: (partly-optional) What I usually do, because I don't want the pencil lines anymore, is first make a new layer and call it "outline". Then I take my tablet and trace over my original line art to give it an "inked" look (this is the optional part). After that's done, I make another layer *below* that one and call it "background".

Step 4: Your original scan? You can delete that layer now if you did the digital tracing step above (if you didn't do that, you would have wanted to promote that to a full layer and called it your outline layer instead).

Step 5: You still can't color in your outlined art w/o destroying those pretty lines, so select your outline layer. Click to properties. In the drop down menu, select "multiply". That's your golden ticket. Now you're ready to color while preserving that outline.

Step 6: Keeping the outline layer on top and the background one on the bottom (which you can fill in with solid coloring if it makes it easier for you to see if you're coloring in solide enough), start adding layers in between. The more you use for each area, the easier it may be to edit or clean up. And do save often, as a pspimage file to preserve your layering (I usually do a layer for each major body area when I'm coloring in a person - skin, hair, clothes, etc).

Step 7: Once you've coloring in the image and cleaned it up, you can finally save it in an uploadable format - jpg, gif, etc - and be proud of your work :) I usually do keep the psp image around just in case I need a version with the layers intact to go back and edit or revamp it at a later date. Saving in jpg and so on will merge all your layers and destroy that ability to color w/o wrecking the outline edges unless you go through the steps above again.