Instructions
Play unranked matches. When searching for a new game in the multi-player lobby, you are asked what type of match you wish to play. Only ranked and some social matches count experience based on your gameplay.
Play matches poorly. If you are stuck playing a ranked match, you can just inhibit your skills to ensure you lose or come in the bottom half during Free For All-type matches. Losing a round will net you no experience points.
Max out your level. Players can earn up to 5,000 experience and become rank 50, General Grade 4. After you reach this level cap, you cannot earn any additional experience no matter how well you play.
Quit out of matches early. Not only will this ensure you do not earn an experience point for winning the match, but you will actually lose a point for quitting early.