I want my students to know that a mistake in math class is not the end of the world and that you can recover from them. Not only can you recover from them, you can learn from them!
To show them I mean what I say, I keep track of the math mistakes I make in class. When I add instead of multiply or I completely botch a problem--we make it a learning opportunity.
Sometimes the students notice before I do and say "Mrs. Stowe made a math mistake" other times I notice and ask them to find what I did wrong.
It hasn't helped everyone, but I do have students willing to say what they are thinking even if it might be wrong. And students are willing to come up to the board, even if they might be wrong. We can all learn from each other when we make mistakes--even the teacher.
Some days are better than others. I made it several weeks without putting many tally marks on the board. But the closer it gets to Christmas, the more I have to add to the board!