When doing NFL Challenge mode, go to the Sportsplex field (the baseball field) and go all the way down to the last challenge. It should say "Sack the QB three times in this play". You can get either a Gold, Silver or Bronze. After you complete the challenge, it should say: "You won of development points", and have the choice to exit or retry the challenge. Choose "Retry Challenge". Keep doing this for however many times desired. When you go back to the main NFL Challenge menu, you will see that all the times you won the challenge, you kept the development points you won. If you did this a lot, you will have a large number of development points to spend on your player.