These days, there are lots of promises floating around the internet to "double your money" quickly. Some offer ways to produce even larger returns on your investment.

If you've been a user of the internet for more than five minutes, you know most of these claims are designed to do the exact opposite. To use a timely example, it should not come as a surprise that buying digital images of monkeys has a very low return on investment.

But there are proven methods to double your money if you're patient enough.

Money plants growing over time.

Image source: Getty Images.

Consider the following:

S&P 500 index funds

The closest thing to a sure thing in the stock market is simply betting on the market as a whole going up over time. In that respect, it has a perfect record.

Despite countless bear markets and corrections, the S&P 500 has always recovered its all-time highs and trudged higher. Thus, by simply investing in a low-cost S&P 500 index fund such as the Vanguard S&P 500 ETF (VOO 0.96%), you're nearly guaranteed to double your money given enough time.

While there's always the risk of investing right before a market crash, the historical data is on your side. Dating back to 1928, the average bull market has lasted more than 1,100 days, while the average correction/bear market has only lasted 194 days.

So even if you buy right at the peak before a crash, all you need to do is wait, and your investment will likely turn out just fine. And if you're smart, you'll keep buying through the ensuing bearish period to make an even higher return.

401(k) employer match

If I could shout it from the mountaintops, I would scream that employer 401(k) matches are quite literally free money.

A financial advisor talking with a couple.

Image source: Getty Images.

Remember how I said the S&P 500 is as close to a guarantee as it gets? Well, the only other thing that comes close is a 401(k) employer match -- when the company you work for matches your retirement contributions dollar for dollar up to a certain point. For example, your employer might match 100% of your contributions up to 3% of your salary. In other words, if you put 3% of your paychecks into your 401(k), your employer will double your contribution.

This is, to my knowledge, the easiest way to double your money, assuming your employer offers this benefit.

Unfortunately, according to advisory firm Financial Engines, Americans leave $24 billion in unclaimed 401(k) matches on the table every year. If your employer offers contribution matching, take advantage of it! It's the easiest double-your-money opportunity you'll ever encounter.

Real estate

Real estate can be a great asset to double your money because it's nearly always in high demand, and they aren't making any more of it.

With the exception of the housing bubble in 2008, home prices have risen consistently for the past 50 years. Part of this can be explained by rising inflation, but it's also due to the fact that housing is a basic need for every person on the planet.

Like investing in the broader stock market, the key ingredient for real estate investing is time. While the near term can be unpredictable, we can almost certainly count on real estate being worth more over time.

The most convenient way to gain exposure to real estate is through real estate investment trusts, or REITs. REITs allow you to diversify much more easily than buying property directly, and they offer much higher liquidity.

For example, aside from investing in residential housing REITs like Mid-America Apartment Communities (MAA 1.55%), you could easily diversify into other sectors of the real estate industry, such as Crown Castle International (CCI 1.02%), which owns, operates, and leases towers for wireless communications.

Interestingly enough, if you compare both of these REITs' stock prices with the S&P 500's returns over the past decade, they track closely.

MAA Chart

MAA data by YCharts.

Boring usually wins

With NFTs, cryptocurrencies, and meme stocks dominating the headlines lately, it's easy to think they are the path to generating huge returns. But as with all speculative assets, the lofty returns rarely last. As a Foolish investor, you should look for sources of long-term and consistent returns like the examples above.