Anthony Gardner on...
First of all you have to get into a good position in relation to where the ball is coming from. The important thing is to get into position to attack the ball.
From there it's all about timing, making sure you jump at the right time to benefit you, preferably to meet the ball at the highest point. That means you can get as much power from the ball as possible.
Once you've got the timing right it's about a good contact and heading it away. Sometimes you have to have a battle with a forward to get there but that's where timing comes into it.
It's about timing the jump because forwards tend to try and put you off more than anything, sometimes they don't even try to win the header and aim to put you off instead.
Focus on the ball all the time. If it's a defensive header you are looking for distance and if you get time, try to aim in an area where your team-mate it. But it's a defensive header, so don't take risks - you want to clear the ball first and foremost.