Start up the game and play Pigskin and choose the helicopter (must have DLC) with the Bird as the pilot (Whirly Bird i think). Grab the football and press 'B' to freeze your position and have a friend do the same right next to you. Now you look at each other and hammer the 'Y' button. This will pass the ball over and over giving you like 10xp a second or so. Very easy to boost you will just have to do it late at night because people will join and start killing you.
Im not sure if we need another play for th enemy team or what but anyway if your interested leave a message.