Work towards upgrades that will allow you to cook more food at once. This is very useful, especially for timed quests. Afterwards, upgrade your tables and grills to allow you to cook more and serve more food at once. You can also upgrade food warmers, which will let you store more food until you get a customer that wishes to eat. The upgrades that reduce preparation and cooking times are also very useful. For example, consider the Cola Dispenser, Burger Fry Pan and Hot Dog Grilling Pan. They will reduce the amount of time required to wait for an item to be ready. After your cooking upgrades, consider upgrading your interior with bar stools, television, tables, etc. The television increases your customers' waiting time. Tables and barstools will boost the tip time. Disco balls will increase the amount of tips that a customer can give.