espn.com has a great story titled How it started: First jobs in football for all 32 NFL head coaches. Below are some of my favorites quotes from some of the coaches.
“I was going to make them need me by working as hard as I could,” Rivera said. “I did everything from taking coaches’ cars to get washed and gassed, picking up lunch. I did all the quality control breakdowns. Basically, from the ground up. I learned what it took to be successful. You had to make yourself indispensable, where they had to have you around to help the team.”