Acknowledge your feelings. Sometimes people allow things like the performance of their favorite team to affect their emotions. Thats okay. If you are angry or upset about your team losing, dont pretend otherwise. Give yourself a chance to vent a little bit, or at least be disappointed.
Keep control. Because its only a game, there is no reason for your anger or sadness to lead to personally destructive behavior. If you are at the game, go ahead and yell a bit, but avoid shouting at other fans (especially fans of the other team). Avoid stirring up fights or throwing things as well.
Eat something. Food can be a great comfort when faced with disappointment, including watching your team lose. Just make sure to eat healthy if you can. People tend to reach for sweets and other unhealthy foods when dealing with a loss. Instead, make sure you are surrounded by healthier foods so that you arent tempted to pack on the pounds after defeat.