As said, don't fall for all the bullsh!t cheap crap that make big claims but deliver sod all. Thankfully those magical 20bhp overfuelling resistors are yesterday's news.
There was a good letter in Honest John's column in Saturday's Telegraph detailing fuel economy of a 406 coupé (think it was a 2.0 petrol). Almost 40mpg with smooth driving and minimal use of the brakes. His wife couldn't get anywhere near because she used the brakes too much.
There's thousands of articles around detailing how to improve MPG - it's probably taken many people until £1.20 a litre to notice their driving isn't up to scratch.
Very much so!