Mine are general and coming from the perspective of working within a shop.

1. No frills for the customers.
You don't want them "hanging out" or meandering around the shop EVER. They need to be in and out and know that there is no bullshitting around.

2. Obtain and stock hard to find parts. These can always make you money when you're not doing bread and butter labor.

These are general things that everyone should know but few abide by. Spend your money on good tools and skip the area for the customers.