I still see deviants asking how to separate their lineart from the background, while there's a very simple way to do it cleanly. I made this action for myself, so that it literally takes one click of the mouse to obtain clean black lines on a transparent background. I decided it was simpler to post it than to keep explaining how to do it!
If you're scanning ink: You must scan your inked piece using the "black and white" (or "bitmapped") setting on your scanner. Scanning in this mode ensures only your inked lines get picked up – anything light grey on the paper such as smudges, unerased pencil, or even lines will be ignored. I recommend scanning bigger than your final size, then reducing it to the desired size after applying the action. I designed the action for this use (scanning inked lines) so this will give the best results.
If you're scanning a sketch: Scan in grayscale and adjust the levels so that the lines are quite dark. If it's difficult to achieve that, duplicate the layer, set the top one to Multiply and merge them. You can do this several times until your pencil lines are strong enough. You'll need to make sure the white background is really white, i.e. clean up any artefacts or they will get picked up with your pencil lines. The stronger and cleaner your lines are, the better the action will work. If there's a lot of grey smudge, that's going to get picked up as well, so make sure the lines are as contrasted as possible.
How to use this: Click Download. Decompress the file in the location of your choice. In Photoshop, open the Actions palette and select Load Actions, then this file [Clean Lineart.atn] Open your scanned file, select the action in the palette and press the Play button. You're done. Your lineart is now a layer floating above a transparent background. (The lineart in the preview looks blurry because it's a screenshot, not because of the action)
If you want to color the lineart alone, just lock the layer and use any coloring tool – only the lines will get colored.
I hope this helps. No credit necessary or anything. Just don't claim you made it – I'd never find out, but you'd still be a prat.
This action was made in CS3, but it works in CS2, so it's worth trying on whatever PS version you have!