This may sound odd, but I avoid using negative language like, "don't".

The rationale is that sometimes when you say something like, "Don't do it this way." There is a chance that they will register "Do it this way."

Instead (and I explain why I sometimes speak this way before I start teaching) I say something like, "This is incorrect", or, "This is a bad example."

Those phrases are less correct grammatically, but less likely to be misinterpreted as well.