Drucker says it best

Peter Drucker is the father of our modern business theories and culture. If you haven't read one of his books, you should. When I read a book I find that I usually glean a few take-aways, but not so with Drucker. You'll love all of what he has to say.

I read this last night from one of Drucker's books and it really provoked me to think about being relentlessly excellent, even as business grows.

The theory of the business must be known and understood throughout the organisation. This is easy in an organisation’s early days. But as it becomes successful, an organisation tends increasingly to take its theory for granted, becoming less and less conscious of it. Then the organisation becomes sloppy. It begins to cut corners. It begins to pursue what is expedient rather than what is right. It stops thinking. It stops questioning. It remembers the answers but has forgotten the questions. The theory of the business becomes “culture”. But culture is no substitute for discipline, and the theory of the business is a discipline.
- Peter Drucker, On the Profession of Management

Don't stop when things seem to be going well. Keep asking the tough questions of yourself and what you're doing as a business. Bottom line - don't get comfortable.