If you don't mind waiting  when you go to fill your car up, then try to buy your fuel from a busy petrol station. While this has not been scientifically proven, busy stations get fuel deliveries twice or more a week. Petrol does 'go off' if let sitting for a long time and its octane rating degrades over time which reduces its quality and therefore it effectiveness.

It is always better to buy fuel when the temperature is coolest, first thing in the morning or later at night is best. This is because petrol derivatives become more dense (heavier for the same volume of liquid) in colder temperatures. Pumps measure fuel by volume (litres/gallons) and not by how heavy the fuel has become. In simple terms, if you fill up when its coolest, you will get more for your money!

When you use a petrol pump, they pump fuel at quite a high rate, so when you shut off the nozzle, fuel does keep coming out for a short time. So, make sure you wait three or four seconds before putting that pump back, and lift the pipe to make sure you get all the fuel in it into your car. You will be paying for what's left over, or what you spill by moving the nozzle out of your tank too quickly, so get it in your fuel tank!

BP Ultimate, Shell V-Power and Total Excellium. Are these dearer fuels worth the money?. Maybe. You should get more miles per tank as the octane ratings are higher. Typically on unleaded, you may get 30 miles more per tank, almost one gallon free. Its up to you to do the math's and test it out. The average improvement in fuel consumption is about 3%. With diesel its not so high, about 1.5% so unless your diesel car is thirsty, its probably not worth the extra. You should see a cleaner engine and increased power, but again not by much.

Fuel Additives /Injector Cleaners. Fuel additives that boost performance are expensive and you are unlikely to gain any more than what he product costs you (similar to higher octane fuels above) and unless you run a high performance car you're not going to notice much difference in performance or economy. Likewise, with injector cleaner. Most fuels (even diesel) already contains fuel system cleaners. It is worth noting that diesel cars can benefit from injector cleaner being used once a year, mainly as replacing injectors in a typical modern diesel can cost as much as £1,000!. You won't notice appreciably better fuel economy, but you may well prolong the life of your fuel injectors and save a huge garage repair bill!