Steaming is the suggested way to reheat. Use a standard steamer, a double boiler or chafing dishes with sterno flames (these seem to work the best for large quantities) and steam for 15-20 minutes.
If you are unable to steam, place in the microwave with moistened paper towels. Heat in batches and in small increments of time. One lau lau usually takes 2-3 minutes in total.