Sheryn’s tip:

why I like digging big holes

Oversized holes like those pictured at the top left (1m by 600mm) can accommodate up to two years of root growth.

If you're using a post hole borer, bore three holes close together in a triangle and then chip away at the centre, so they become one large hole. It's much quicker and easier than digging with a spade. Roughen the sides to allow root penetration.