Great question. There's enough to be done on the course learning how to talk while flying accurately, without having to catch up on stuff as well.
I'd say any of the following would help:
Learn by heart the POH, all the power margins (both take-off and landing), and autorotation speeds. Make sure you can explain to a friend every diagram in the POH, and how you would react to every emergency and why.
Revise so you can explain fluently to a friend how dissymmetry of lift, inflow roll, flapping to equality, flapback, and LTE work. There are some great theory briefing notes on
Helitutor.
Practise drawing a complete frontal system from the side from memory, and explain to a friend what weather you'll observe from the ground as it passes.
Find out which set of briefing notes you'll be using on your course, and use them to sketch your own version of all of the exercises so you're familiar with the syllabus.