I've found the best way to spend less on fuel is just don't drive lol, nothing else works as well! 
Pretty much what has been said though. Except keeping off boost, boost increases efficiency as Chris said. I tend to stick at 55mph (until I get fed up/panicky and creep to 90+ forgetting about mpg lol). I find 60 is annoying as you have to keep overtaking lorries - slowing behind them, then accelerating which is just throwing fuel away.
Pretty much what has been said though. Except keeping off boost, boost increases efficiency as Chris said. I tend to stick at 55mph (until I get fed up/panicky and creep to 90+ forgetting about mpg lol). I find 60 is annoying as you have to keep overtaking lorries - slowing behind them, then accelerating which is just throwing fuel away.