Using a large re-sealable plastic bag, pour the margarita mix and add the chicken breasts. Then press out as much of the air as possible before sealing. Place in the refrigerator for 4 to 8 hours to marinate. Preheat an grill for medium-high heat. Spray some nonstick cooking spray onto the grill grate. Remove chicken from marinade, and discard the marinade. Grill chicken 8 minutes per side, or until juices run clear.