This can seem fairly difficult to get out of - largely because people start to panic when somebody has their hands clasped around their neck and start doing incredibly ineffective things like trying to pull their hand off by grabbing onto the attacker's forearm. There are a lot of fairly simply ways to get out of a frontal strangehold that don't require a massive amount of training or strength.
1) Arm over
Lift a straight arm up and over your attacker's arm (so that their arm ends up in your arm pit). To do that, you'll have to turn a little, and force their arm downwards in the process. Continue forcing their arm down until they have to let go - you'll have your bodyweight behind this manoeuvre so it doesn't require you to be stronger than your attacker. Your natural position at the end of this technique will lend itself to a backfist or knifehand strike.
2) Strike to temple
Just because you're not in a dominant position doesn't mean you should stop attacking - in fact, that's one of the worst things you could do. Make a fist, but instead of curling the thumb over your index and middle finger, bend it so that the knuckle of the thumb sticks out of the side of your fist, and the end of the thumb touches the first knuckle of your index finger. Use that thumb knuckle to strike your attacker in the temple - hard. It's generally best to do this with a straight arm (and you'll probably have to to reach).
3) Strike to inside of elbow
Joints are always a good weakpoint to aim for in a self-defense situaton - whether you want to bend them the wrong way to break, or minimise your attacker's reach advantage in a frontal strangle. It's is particularly effective if you can't reach to do a temple strike or eye gouge. If their arm is outstretched, simple use a ridgehand to strike to the elbow, forcing it to bend. A natural follow-on from this technique is to twist to the side slightly and then use a knife-hand strike to the throat.
4) Drop and strike to knees
Another technique that goes for joints. Hold onto your attacker's wrists with both hands, and drop down. Use a low front kick to strike at your attacker's knee - go for the inside of the knee, right on the joint, and hit hard.
5) Eye gouge
Attacking eyes is one of those techniques that will work on pretty much anybody - protecting the eyes is a natural reaction, and an attack evokes the same sort of panic as a choke. Use a spear-hand to strike at the eyes - it doesn't need to be particularly hard to be effective, but it helps.
There are a myriad of other techniques you can use - jamming your keys into your attacker's wrist if they're handy, pinching them hard in the armpit or striking to the armpit, kicking to the groin etc. Which is these techniques works best will really depend on your situation - if your attacker has a longer reach than you, then 2 and 5 are not options. If there are a number of attackers, you may not want to use 4 as you will end up on the floor.