One trick that I found to pop out the lines on
scans is to load it up in photoshop and turn up
the contrast till the lines are dark.  The
problem is then that it also pops out all the
lil bends and smudges on the sketch and makes the
sketch look really dirty and messy.  What I did
then was to set the fill color to white and use
the paint bucket to fill in the messy background,
but the lines stay dark.

Below is an example of a sketch I did this on. If
you look closely, you'll see that the interior of
the figure is blue-ish and a lil smudgey while
the exterior of the figure is clean and white.
This is because I only used the fill tool on the
exterior.  I thought it gave it an interesting
effect not to and was easier and quicker than
filling in all the interior areas, but your
mileage may vary.