Once you have your image scanned, open it up in Photoshop.

Go to Image > Adjustments > Levels...

Here you have the Levels HistroGram window.

By adjusting the highlights tab (3) in the levels histoGram to the left, you'll be able to pretty much 'erase' the lightest pencil sketches while preserving much of the darker ones.

You can also darken the clean lines by sliding the shadows tab (1) to the right.

The midtones tab (2) can be used to level out the details between light and dark lines.

If you have the Preview option checked, you can see your image in the background, and can make the appropriate adjustments.

If you're satisfied, click OK.