Ok, this might be hard to explain but hopefully not.
Once I've scanned in the drawing into Photoshop I will use 'Levels' to adjust the contrast and get rid of all of the bits of 'dust' or whatever you call it so what I am left with is just the black lines of the drawing. Then I double click on the bg layer in the layer menu and press ok. This unlocks the layer that the drawing is on. Then I go to the 'select' menu and use 'colour range'. Once this separate window opens up make sure that the colour you are selecting is white and put the 'fuzziness' up to 200 and press ok. Then press delete, then de-select, then use the 'levels' to darken everything (selecting and deleting all of the white makes the black lines slightly lighter around the edges. Whacking the levels all the way to the right twice will get rid of this) and there you go - your line drawing is separate from the paper.
This is what I use on pretty much every drawing as it allows me to easily work behind the line drawing. I'm sure there are other ways of doing this but I find this to be a very quick way. I have added 'select colour range' to my shortcut menu which helps to speed the process up. I hope this makes sense and I hope this is what you meant. If any of this is confusing and doesn't make sense please let me know.