I've been researching this question too. I've spoken to my most financially sound friends and lurked in personal finance forums.
The answer always seems to be: having one or more passive income stream. This could be from stocks, property, owning your own businesses, etc.
So I figured out a plan for anyone who wants to become relatively well off:
Be frugal and save as much as you can. If you have liabilities such as debt, clear these ASAP. Minimise what I call 'holes' (any expense that bleeds money from your savings
Once you have a nice emergency fund, use your saved up money to buy your assets
Repeat as necessary
We will never be as wealthy as the Waltons or the Johnsons (those with intergenerational wealth) but it's possible for a normal person to retire a millionaire!