Know what your expenses are and how much you are spending. Know which of the things you spend money for are really important to you, and what you wouldn't actually miss that much. Know how to keep your spending in check as your income grows, not just increasing spending because the money is there.
People tend to react emotionally to the teachers of these principles, like Dave Ramsey or Mr. Money Mustache. But it's not personal. It's just math. You can choose to cut way back on your spending if you want to retire really early, if that's what's most important to you. Or you can choose to spend more extravagantly and retire later (or not at all).
Or instead or retiring early, maybe you want to live really frugally but keep earning as much as possible, so you can give as much as possible. The underlying principles are the same.