How to Escape the Headlock

The headlock is one of the most common holds used both by untrained and trained fighters.

How to avoid the headlock

Be aware of your surroundings. If you are aware of someone approaching you, it is unlikely they'll be able to apply a headlock, and if they try you can defend against that similarly to a shoulder grab since you'll have enough distance to start with. Having good posture is also very important.
If you've only started to be aware a split second before the headlock is applied, it might be possible to maneuver out of it by ducking and blocking his arms. (In any case, the longer you wait, the harder it will be.)

Tuck your chin. You do not want them to cut off your supply of air or blood. Turn your head towards their body, using their chest to protect your face from their fists. Grab their hands grabbing each other and the hand that is choking you and pull the hands down to release the pressure
Keep a stable base. This means stay low and keep your legs relatively wide in a horse stance position with your legs bent. They will probably be thrashing you about and dragging you by your neck, so the ability to gain stability and counterattack at a moment's notice is crucial.
To counter-attack, there are many possible choices, depending on what position you're in at any one moment. A particularly effective one is to punch their upper inner thigh or their groin. Afterwards pull their head area backwards using any hold you can get (e.g. hair, collar, eye sockets). Then push their arm away and run.
Another option is to take your opponent down, as is demonstrated in the next section:
There are many different defenses against the headlock and many different ways to do the headlock.

Here we see the classic headlock

Step 1
Rotate your body, use your shoulder and place your arm in front of their body

Step 2
Place your leg behind both of their legs

Step 3
Fall backwards and trip them over your leg

Step 4
Push off the ground and run (or fight)

Use your free arm to protect your face from punches.
The first movement required is much like the exercise where you stand with your legs apart, bend over and touch your right foot with your left hand and then your left foot with your right hand. Once your shoulder is free reach around the front of their body.
As you lean backward to pull them over your leg also push them with your arm, digging your elbow into their chest.
Your head could land on their arm but just in case lean your head forward to avoid hitting it on the ground. Try to shift all of your weight onto your elbow.