Try Not To Use Air-Conditioning

Having air-conditioning is a nice luxury, and when it is baking hot air-con seems like the greatest invention ever! However air-con does increase your petrol consumption by as much as 10%, so when its warm just have your fans on cool instead of having the air con on, or just have the window down slightly. If this fails to cool you down then try to just have the air-con on in short bursts.

If you are traveling at over 60mph and it is too hot then it is recommended you actually have the air-con on instead of putting the window down, this is because having the window down increases drag which in turn reduces your fuel consumption.

Problems
If it is really hot then don't leave the air-con off just to save a couple of pounds! So sometimes you can't get round it, but most the time just have the window down slightly.