Not just knowing how to negotiate, but a better understanding of the comparable salary for that position and geographic location. It's difficult some times to appreciate what you're worth when starting out (or restarting) and granted - early on you may or may not be worth that much.
Case in point - I relocated years ago and the salary that I negotiated was comparable to the salary that I'd left in a previous locale. What I didn't grasp at the time, was the increase in cost of living for the new location. In reality, while I had negotiated an increase in pay for the new job, I'd actually taken about a 10% cut in pay based on cost of living.
Granted, people have to learn from their mistakes. And true, it may not be very common to relocate as part of your first job. But, with all of that said, I would say early on in your career, understanding what you're worth, and how that is compared to where you'll be working/residing is important. It's definitely something I'll have to help teach my children as they come of age.