* Food: Upgrading your Food items will yield you more coins, put simply. Since your score in each episode is dependent on the tips you make from your hungry customers, upgraded Food items are sure to (b)raise your score!

* Appliances: Upgrading your Appliance items will help you prepare not only more food, but food made faster -- as each upgrade to a given Appliance speeds up its prep time and/or opens a slot, such as another oven burner, fryer slot, etc. Sometimes both in one upgrade!

* Decor: Upgrading your Decor items (walls, floor, seat appearance, etc.) will make your customers much more patient.  If you have a lot of customers losing patience before you're able to prepare their food, you might want to look into purchasing some Decor upgrades to chill them out!