Debbie and Mel shared a wide range of ideas, all of which are visible through their slideshow if you click on any of the images.
These two teachers use 'warm, hot and scorching' to rate all tasks and activities in class; this way, the onus is always on the students to judge the level of task they're willing to push themselves to. The teacher could also suggest which level of task each student should be aiming for.
Debbie and Mel also shared other ideas and frameworks which all help towards pushing students to higher levels of thinking and discussion.