Well- the easiest way to answer that is with one word- SERVICE. ...What?
Give the BEST customer service. Always make your customers jobs and lives easier. Always seek to make your customers feel valued and important, like they're your priority.
This does two things, it makes you irreplaceable with your current customers and it makes them brag about you to they're friends and colleagues. Which equals free advertising for you and you're already going in a step above your competition! (Because your current customers are huge fans of yours.)
So, there it is- a million dollar education all for free.