First, don't assemble all of the sandwiches if you think there will be leftovers. It's best store the chicken, Cole slaw, and buns each in a separate airtight container, in the refrigerator. When you are ready to eat the leftovers, microwave the bbq chicken until hot and then assemble the sandwiches. You'll likely need to add a little extra bbq sauce as much of it will have been absorbed by the chicken. Leftovers should be eaten within 3 or 4 days.