Escape being buried alive?

Does anyone know of the best way of going about getting out of being buried alive?

As in, once you're already in the coffin, 6 feet under, how would you suggest getting out (assuming there is no horn like those used when getting buried alive accidentally was actually a concern)?
Update: In Kill Bill she would have be crushed alive by the weight of the dirt coming down on her.

That's what I'm watching now and how I came up with the question.
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