Fill the radiator and check the coolant
This is a no-brainer. The radiator helps keep the engine cool, and in a heat wave -- with warmer air trying to do the same job -- it's crucial to make sure the radiator is full and carrying good coolant.
Carry spare water
Have at least two litres of water spare in the car, in case you do overheat the radiator. Even a couple of litres to top up with could help you get on the road again. And if the radiator boils, don't open it while it is still hot -- it will gush the remaining fluid out, and could scald you in the process.
Make sure your tyres are properly inflatedAn underinflated tyre gets hotter, and on an even warmer road surface in a heat wave, there is increased risk of having a blowout.
Keep your fuel tank as full as possible
Having fuel around the fuel pump helps keep it cool and running smoothly. Make sure the tank is at least more than half full.
Use sun deflectors and leave windows down a littleIf you can deflect sun off the windscreen and safely leave your windows down a couple of centimetres -- even in the shade -- you will help the interior stay just that little bit cooler.