One of my favorite legal blogs or blawgs -- http://www.illinoistrialpractice.com -- recently posted these five rules from Scott Turow to follow when cross-examining witnesses:
1. Never ask a question to which you do not know the answer--unless it doesn't matter, or you have nowhere else to go.
2. Always listen to a witness's answer before asking your next question.
3. Never ask the one question too many that will allow the witness to explain away a damaging answer he's already given.
4. Forget Perry Mason. The purpose of cross is not to win the trial at once, so much as lay the foundation for closing argument, or for the testimony of other witnesses.
5. Know when you've accomplished enough and sit down.