This is a subject that often pops up on Photoshop forums. There's no one "hard and fast" way to do it - like most procedures in Photoshop - although most seem to use Levels as Dean has suggested. Here is a selection of links to tutorials that may help (including one right here at BioRUST, and a prehistoric one by graphics guru Kai Krause that has screenshots of Photoshop 2 - although I think the technique is still valid):
BioRUST.com :: Tutorials >> Cleaning Scanned Lineart
Cleaning Scanned Images in Photoshop - Associated Content
Scanning and Preparing Lineart and Drawings into Photoshop - Associated Content
CleanUp Fax&Scans: Kai's Power Tips: The Gurus Network