Sternos should be lit and hot water should be placed in your Chaffing Trays about a half hour before serving time. When setting up remove lids from the sterno cans. Think about renting an URN just for hot water and rent a metal coffee pot also.
Appropriate serving utensils should be placed in front of each tray. (Many caterers do not provide utensils. Be sure to ask!) Butter, salt, pepper and any other condiments should be ready and in place. Don't hold a line up with hard butter. Take it out a few minutes early to soften. Order rolls sliced or slice rolls in advance to keep your buffet line moving. (Cover with plastic wrap to keep fresh and remove just prior to opening buffet). Anticipate requests such as Parmesan Cheese, Horse Radish etc. Have it readily available, with a spoon.
Think about how your dinner will be cleared. If guests are to dispose of their own debris make it convenient. If you hired wait staff, discuss this in advance.