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How to achieve financial independence

It comes down to how you save, spend and invest. Maximize your saving -- many of these people figured out how to save 50 percent of their incomes or more, sometimes by purchasing multifamily homes and taking on enough tenants to make an income.

But the spending side is key as well. The author of the website 1500 days, Carl J. set a goal for himself to bring his assets to $1 million in that amount of time. On his site, he details the step he took to reach his goal: Being financially independent and finally leaving the job he despised by April of this year.

Carl readily admits that he was making a good salary (around $100,000) and that he had been maxing out his savings in a 401(k) account account along the way. So he was almost halfway to the goal. But he managed to stock away a significant amount of money and achieve his objective by methodically tracking how much his family was spending and cutting away waste.

"I had no idea how much money we were spending," he told CBS MoneyWatch in an interview. "So we did start keeping track of our spending ... I opened up a Google doc and set up categories, going out to eat, bonus things like alcohol, vacations."

By cutting out excessive spending and revamping their lifestyle, he and his wife were able to save a significant portion of their incomes each year and invest the money instead.

Financial adviser Kyle Mast said he has more and more clients come in to ask about achieving financial independence earlier in life, and while it often involves being more creative to make it happen, it can be done.

"Have a high savings rate, low living expenses ... there's all kinds of ways to travel for free, save money on things like cell phones, just by doing a little bit of research," Mast told CBS MoneyWatch.

Invest to build your assets

But saving will only get you partway to your goal. It's crucial to make your money grow to the point where it can sustain you.

"Save as much as you can, but investing is hugely important and [you need to] figure out how to do it correctly," Carl said. He taught himself how to use personal investing apps as a way to increase the money he was saving.

Financial adviser Lucas Casarez said if people plan to retire early, it's important to do their research and realize how long they'll need that money to last. Remember, you don't want to outlive your retirement savings.

Be aware that if you're under 59, you won't be able to take money from an account like a 401(k) without paying an early withdrawal penalty.

For those withdrawals, Casarez recommends using nonretirement investing accounts or a retirement vehicle like a Roth IRA, from which the initial contributions can be withdrawn anytime, tax-free.

"Roth accounts are really fantastic tools as far as flexibility," Casarez told CBS MoneyWatch, "Any contributions to a Roth account can be withdrawn without penalty -- though you can't take out the earnings" without penalty.

That said, it's still a good idea to max out your 401(k) contributions while you're working. If you are saving for FI/RE, that means up to the $18,500 annual limit, not just meeting the employer match.

He cautions that with other types of investing accounts, "Try to mitigate capital-gains taxes. Know what's going to be considered income."

Challenges to consider

Keep in mind, even when you reach financial independence, it can be helpful to continue working part-time in your current line of work or take on a side gig to maintain an extra income stream or access to health insurance.

Health insurance is a big consideration. You won't be eligible to register for Medicare until age 65, so if you leave your job, you may need to purchase insurance in the private market -- which can be much more expensive than a plan you get through your employer, who helps shoulder the cost.

And though the idea of being able to walk away from a day job and not work again may sound like a dream come true, some financial experts caution people to take care when retiring from work completely, especially at a young age.

"The challenge is that if you're going to retire at age 40, you'll probably live until you're in your 90s and you're going to be pretty active," financial adviser Ryan McPherson told CBS MoneyWatch. "Work provides a tremendous amount of structure, social interaction, challenges and a sense of accomplishment," he said. "You can get bored and depressed very easily. Keeping yourself involved in full-time work or volunteering is very important."