Join 500,000+ artists on ConceptArt.Org.
Its' free and it takes less than 10 seconds!
The title pretty much sums it up. I tried scanning a few sketches into Photoshop, and they hardly showed up. Is it the scanner, darkness of the sketch, what?
Here's a really old thread that might help you.
Has some good info:
Would the general consensus to be to ink/darken the primary lines before scanning?