How to minimise the amount of concrete needed for a shed foundation?
If you'd like to minimize use of concrete, don't use any. I have 4 sheds that have been sitting for 14 years on sections of pressure treated wood, set on top of the ground - and a 5th that's on 4 pressure treated wood posts set into the ground. No concrete at all. Wooden floors (not pressure treated - only the ground-contact wood is PT.) 

Nice and dry.