Only Fill-Up In The Early Morning Or When The Ground Temperature Is Cold
Remember that all service stations have their storage tanks buried below ground. The colder the ground the more dense the gasoline, when it gets warmer gasoline expands, so buying in the afternoon or in the evening....your gallon is not exactly a gallon. In the petroleum business, the specific gravity and the temperature of the gasoline, diesel and jet fuel, ethanol and other petroleum products plays an important role. A 1-degree rise in temperature is a big deal for this business. But the service stations do not have temperature compensation at the pumps.
 
Do Not Use The Pump's Fast Mode When You Fill-Up
If you look you will see that the trigger has three (3) stages: low, middle, and high. You should be pumping on low mode, thereby minimizing the vapors that are created while you are pumping.  All hoses at the pump have a vapor return. If you are pumping on the fast rate, some of the liquid that goes to your tank becomes vapor. Those vapors are being sucked up and back into the underground storage tank so you're getting less value for your money.

Fill-Up Your Tank When It Is Half Full
The reason for this is the more gas you have in your tank the less air occupying its empty space. Gasoline evaporates at a very fast rate. Gasoline storage tanks have an internal floating roof that serves as zero-clearance between the gas and the atmosphere, so it minimizes the evaporation.

Do Not Fill-Up When You See The Fuel Truck At The Station
Another reminder, if there is a gasoline truck pumping into the storage tanks when you stop to buy gas, DO NOT fill-up; most likely the gasoline is being stirred up as the gas is being delivered, and you might pick up some of the dirt that normally settles on the bottom.