Upgrading your quarterback's passing accuracy and distance is a smart thing to do at the begining. The ability to make big plays consistently will earn you more touchdowns and - thus - more experience points.
When on defence, you can move players around to new positions before the ball is snapped. Be sure to adjust your team's positioning if you've picked the wrong line-up to counter your opponent's play.
Upgrading strength allows you to break tackles more easily. Be sure to make your running backs as strong as possible.