It depends on the snake. Obviously a fully grown retic bite would be more of a concern than a Ball. If it's a Ball, just wait it out. They'll release you eventually and push the head forward a little when they do start to release to prevent the rear facing teeth from coming out.
For large snakes, I've heard of a number of different techniques such as running their head under hot/cold water, spraying alcohol on their head, and even bending their tails. Fortunately, I've never been in that situation so I couldn't tell you which one is the most effective method.