It can happen to everyone. Mishka Yaponchik used such a method. Man is buried alive to scare or to get rid of him. If second – you can only hope for yourself.
1) Do not waste air. In the classical coffin there is a limited air supply – for an hour or two. Make a deep breath and breathe out slowly. Making a breath, do not swallow, it causes hyperventilation. Don’t illuminate the coffin using matches or lighter, because it takes away oxygen. You can do it with a pocket flashlight. Don’t scream. It will only make worse: heartbeating and palpation fasten and it takes more air.
2) Try to shake the coffin lid. If it of a bad quality you can even make a hole in it with your ring or belt buckle.
3) Cross your hands on your chest, put palms on your shoulders and take off the shirt. Tie it in a knot over your head. It will protect you from choking, when the ground falls to your face.
4) Try to push the coffin lid with your feet. If it is not expensive, you will easily break it. They usually brake under the weight of a ground after they are buried.
5) As soon as you break the lid, push the ground from your head to feet. When there would be no place – try to free the space around you .
6) Whatever make an effort to sit down. The ground would fulfill empty space. And for God`s sake, don`t stop. Breathe calmly.
7) Get up! Remember that the ground in a fresh grave is always loose and getting out from it relatively easy. It is much more complicated to get out when it rains: wet ground is more dense and heavy. I can say the same about clay.
Well, good luck to you!
I do not remember where I borrowed the text
By the way, they made a very realistic film with the same name recently.