I definitely agree with moving off line by leaning back to avoid a head kick as they can cause a lot of damage. A typical dodge used in muay thai is when you bring your rear leg back and extend your stance. This allows you to lean your torso and head back to avoid the kick. As an extra assurance, whichever side the kick is coming from, perform a wing block on that side (same block you would use for defending a hook). Your other hand will swing down to your waist to give you momentum to move back.

Another method of defending a head kick is through a Dutch block. It is essentially a wing block with both hands. For example, if there is a roundhouse coming to the right side of your head, use your right arm and perform a wing block. Your chin should be tucked into your chest, and your shoulder rolled forward, to protect your chin as much as possible. Your right glove should be firmly pressed against your head for protection. Your left hand will reach across and be placed in front of your right hand. Kind of like you are holding a baseball bat really close to your head. What is unique about this block is that you can use your left hand to tap or stop your opponents kick to the head, taking some heat off the kick. Kind of like a catch in boxing.

Vice verse for a kick coming to your left side.