It seems like almost all people I know, have a tight neck. Yet most of them ignore it and do nothing about it. Remember if you don’t use it, your body will forget what the normal would be. Your body needs a maintenance regularly like your car or a bike.
How did tight neck happen?
Normally when the muscles around the neck are shortened and kept for so long. Reading a book, using a cellphone, laptop, writing, all of these make you be in slouching position, have forward head position. If these muscles are shortened, we need to stretch them to lengthen.
Never stretch when your body is cold
I am sure everyone heard of this. Not only is this not as mush effective as when your body is warm, but also you may injure your muscles. Especially the neck muscle stretch involves cervical spine movement.

I learned this stretch from a friend of mine who is RMT and it works well for me.
Sitting down on a bench, and hold the bench with one of your arm.

After that, the head tilt in the opposite direction from the hand that is holding the bench. Do not stretch too quick but slow and gentle. Hold the position for at least 30 seconds.
If you are to stretch your tight neck without a bench

Pretty much the same idea as one I explained above, but you use your hand to pull the other hand down, then start tilt the head in the opposite direction. Hold the position for at least 30 seconds.
At the end of Personal Training session
I normally help stretching my clients last 5 minutes of personal training session including the upper trapezius muscle. You may feel better after moving around plus partner stretching!!
Click here to learn
some other mobility/stretch exercises
Some exercises during personal training