A tip from "A Snipe-Hunter's Guide to Coaxing Answers from Hostile Witnesses," by Judge Ron Spears--
From the beginning, keep a separate folder on each potential witness and maintain a list of facts that witness may be able to testify to and how it could help or hurt your case. Craft questions throughout discovery that will fully elicit the adverse witness's knowledge on those important issues before trial and especially during depositions, always with an eye to use at a later proceeding.
As Spears points out, "You cannot impeach with a question never asked or an answer never given."
The full article can be found in the May, 2009, issue of the Illinois Bar Journal (available online only to ISBA members).