To prepare students for the clarion register, we work on what we call rockets. Play a note in the chalumeau register then add the register key. My progression is C-G, Bb-F, A-E, G-D, F-C, E-B, then moving up C-G, D-A, E-B, F-C.

Paula Corley's book "So You Want to Play the Clarinet" has a page dedicated to this transition that works wonders with my kids. Crossing the break isn't hard if you present it correctly.