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That decision is to pay yourself first.
"What most people do when they earn a dollar is pay everyone else first," Bach explains. "They pay the landlord, the credit card company, the telephone company, the government, and on and on.
"The reason they think they need a budget is to help them figure out how much to pay everyone else so at the end of the month - or the year, or their working life - they will have something 'left over' to pay themselves."
This mindset plagues many of us, and is "absolutely, positively financially backwards," he emphasizes. "And because this system does not work, Americans wind up trying some pretty strange ways to get rich."
These routes to wealth include: winning it, marrying it, inheriting it, or suing for it. But there is only one proven way to get rich, Bach writes, and it also happens to be the easiest way: "Nothing will help you achieve wealth until you decide to pay yourself first ... [It] means just what it says. When you earn a dollar, the first person you pay is you. Most people don't do this."
Bach recommends starting simple, by putting at least 10% of your gross income into a pretax retirement account, such as a 401(k), Roth IRA, or traditional IRA.
If you're not comfortable making a 10% contribution, it's better to start small than not start at all. In fact, Bach started by setting aside a mere 1%. Over time, he gradually increased his percentage - from 3% to 10% to 15% - until he reached 20%.
Once you've decided on a percentage to contribute, make it automatic, Bach stresses. This way, you'll never even see the money and you'll learn to live without it.
"You'd be amazed how effortlessly you can learn to live on a little less," he assures. "You can't spend what you don't have in your pocket."