This is what I usualy to clean up an scanned drawing:
- Desaturate the image
- Slightly increase brightness/contrast (don't overdo this step, otherwise you will end up with horrible crispy lines)
- Manualy clean things up with a hard white brush (very important, don't be lazy on this step)
- Duplicate the layer and set the top one as "multiply" and 50-70% opacity. (This will make your lines darker. You can also use a slight blur pass on the top layer if your lines are too crispy).