The age you start saving, your annual savings rate, and staying disciplined are the keys. Going heavy in equities is also important.
I started saving at age 25 and put away at least 20 percent a year into my 401K, including company match. That fact that my employer matched me dollar for dollar up to 10 percent helped. About ten years into my career, I upped my contributions slightly to hit the maximum allowed the IRS. Additionally, my wife and I have made Roth IRA contributions up to the maximum amounts since 1998.
We managed to save some money in non-qualified accounts, but the vast majority is in tax deterred and tax free accounts.
It really comes down to paying yourself first. View your savings as a mandatory expense, just like a mortgage or car payment. If you have your savings taken directly out of your salary you, will learn to live with what’s left over, trust me. Obviously, this is easier with a 401K. However, you can do the same with a Roth IRA by auto-depositing money into your account each month.