A Thorough Understanding of the Other Side
If there is one thing I got from law school, aside from a few emotional scars, is that there is another side to a case and you'd better find out what it is. A healthy skepticism is developed. You're citing an expert? The other side also has experts. You've got a study? Ditto. You have a small army of witnesses? How credible are they? Tell a lawyer that a matter is an open-and-shut case and you'll see a slight smile. They don't take things at face value. They cannot afford to do so.
This skepticism is also developed in other jobs. Tell a literary agent that you've written another War and Peace and watch as her eyes narrow. Tell a military officer that a complicated operation will run like clockwork and you'll be lucky if he doesn't guffaw. I can't recall who said that the more you know about a topic, the more conservative you are about it but that opinion was right on the mark.
Do your homework. Seriously study and argue the other side. If it seems too weak, you're probably missing something.