There are a few ways to approach trying to punch someone in the neck. The easiest is if they are not expecting the neck punch, there attention will be elsewhere and their hands will likely down. I do like the "look over there!" move as illustrated in the photo to the left, but I think that might only work when you are 12 years or or fighting someone who is drunk.
Another approach is to use the neck punch in a counter or as part of a flurry of strikes. Your opponent might be more worried about connecting their attack or protecting their face and this can allow a punch to the neck to slip through. There is a technique that is part of the Wing Chun system, which gave Bruce Lee is start in Martial Art, that defends against a punch and opens up your opponent to a neck punch as your counter. In Cantonese it is called "Bil Sao" and is basically a block where you connect with your opponent's upper forearm (close to the inside of the elbow) and you push their attack up and away. This move can be made with an aggressive step toward your opponent or a retracting step to create distance or angle. While one are is blocking a punch the other is position perfectly to strike any target in the center of your opponents body, for example the neck.
There are plenty or variations of this and the defense and strike can be applied in a number of ways. Below is a video that shows another variation of the counter strike to the neck. I personally disagree with this guys use of a cross are technique because it leave you open to a punch from the opponent's other hand, but you get the idea o how you might put together your own opportunity to use the magic of surprise.