Classic teams:
Defeat the teams in classic challenge mode to unlock then in single player mode.

Successfully complete the Running Drills in training camp mode to unlock the 1977 Dallas Cowboys in single player mode.

Successfully complete the Passing Drills in training camp mode to unlock the 1996 Green Bay Packers in single player mode.

Successfully complete the Kicking Drills in training camp mode to unlock the 1964 Cleveland Browns in single player mode.

Successfully complete the Tackling Drills in training camp mode to unlock the 1983 Los Angeles Raiders in single player mode.

Getting players in dynasty mode:
Use the following trick to get any player and create a player with as high a rating as you want in dynasty mode. First, at the main menu, go to "General Manger", and trade, sign, or create whomever you want. Next go to "Season", and on to "Dynasty". Turn off the salary cap and alter your depth chart. You should now have all of your favorite players without having to pay too much for them