Hmmm things every supervisor needs to know, well this is my list based on most important first:
Use of the SVR menu (Spot check, Telco Check, refunds, RTC, etc.) and how to do these CORRECTLY
When/how/who to call for Priority 1's
How to plan for lunch breaks and tea breaks
How to answer the phone
How to do Britex/Rugdoctor on storenet
Using office (EJ, Register Over/Short, Unrecalled Save trans, Force sign)
How to use the EFT fallback system/doing manual EFT
Gap Scan, Negatives and other smokeshop inventory on ISIS (if you dont have a separate smokeshop assistant)
Other office reports (e.g voids, refunds) if needed
Making ad-hocs
Corecomms
Everyday Money
Using StoreNet
Aaaand thats all I can think of if you want them to be trained up well, other than that I think being a good supervisor just takes a lot of practice.