Get to know the prices in your area. Prices will rotate around the average by as much as 5 cent per litre, so its worth getting into the habit of noticing prices as you drive arround.
Buy petrol in units of litres rather than in units of Euros. Most people buy 10euro 20euro or just fill up - this disguises the value that you are getting. If you always buy 30 litres of petrol rather than 20euro worth, it becomes very obvious when one garage charges 26.70 euro (89 cent per litre) and another charges 29.70 euro (99 cent per litre) for the same amount of fuel.
Ignore loyalty schemes, bonus schemes etc and just concentrate on price.
Use fuel wisely. Your driving style will greatly affect your fuel consumption. Drive gently, avoiding harsh acceleration and braking. Roofracks add to fuel consumption so remove them when not in use. Air conditioning - standard now on more and more cars - adds greatly to fuel use, so turn it off when you dont really need it.
