The pen tool.
It's a long, annoying process, but the more you do it the faster you get at it. If you don't ink your pictures it's sort of the best way to get clean lineart (I find, at least).
Essentially, it goes like this:
1. Scan the sketch.
2. Create a new layer.
3. Set the brush tool to whatever you want your lines to look like; for example, if you want thin, crisp lines, use a 1 pt black hard brush with full opacity.
4. Select the pen tool.
5. Use it to trace a line on your picture. Just do segments at a time; it's a real bad idea to try and do the whole thing all at once. If you're not familiar with the pen tool, this could take some time to get good at.
6. When you've traced a line, right click, click on "stroke path", click "Okay"/push enter, and finally, push delete.
7. Repeat until you've got some lineart.
8. Color underneath the lineart, above it, or on it, if that's your game.
