I just made up a wee photoshop action for the technique I use for preparing finished lineart for colouring. This technique works equally well whether you're using stark black & white finished inks, or full greyscale pencil/paint artwork, but it's important the art should not be coloured at all. [EDIT] just in case anyone else misunderstands, this action is intended for comic colourists and digital painters.

Copy this action into you Photoshop Actions folder, then open your artwork and make whatever basic value adjustments you'd normally do (I prefer levels, but other folk do things differently). Then just run the action - you'll end up with 4 layers - (from the top) lines, tones (empty), flats (white field) and the background layer (switched off). The background is the original artwork, so you can always refer back to that if you screw anything up.

The beauty of this technique is that the linework is actually a layer mask - it allows you to paint colour on the black (left-hand) field of the lines layer, so you can do all sorts of interesting colour-hold effects (colouring the lines), without ever actually interfering with the lines themselves. If you're not happy with the colour hold, just paint that part black again. Just be sure you don't do it on the mask part of the lines layer!

I made the action in CS2, but it seems to work fine in PS7 too - let me know if you get it working in any earlier versions.