So when someone like me pipes up to say he's got a way you can virtually guarantee to double your money, you're pretty sure to sit up and listen.

Now I've got your attention… a quick word of warning.

Doubling your money does NOT involve investing in risky mining stocks. It does NOT involve algorithmic trading, or charting, or buying gold.

It does involve skill, time and patience. We can help with the skill. But in a world where most people want to just get rich quick, time and patience is down to you. Read on for more…

But first, to the markets. The ASX is back on the up today, fired up by energy stocks.

Damn it.

I should have bought some oil stocks yesterday.

Overnight, for the first time in eight years, OPEC agreed to cut oil production, a move that drove the oil price up 10% to more than $US50 a barrel.

On Wall Street, energy stocks surged, at the expense of technology stocks. It's a similar story here, with ASX gold stocks also being sold off. Those poor old gold bugs must be crying into their nuggets.

So it goes at the big end of town. With most institutional money effectively fully invested in the market, you have to sell something to buy something else.

Not so for many retail investors. Sure, we've got money invested in the market. But we also sit on cash.

Why so much cash?

It's safe. When markets fall, cash doesn't.

It also affords us opportunities. To top up on existing holdings. To add a new holding to our portfolio. To buy shares if and when the market corrects, or even crashes.

Today marks the first day of summer, and the start of the silly season.

Not Christmas shopping. I'd rather have my teeth pulled.

December. A time to take stock of 2016, and look ahead to 2017.

So far this year, the S&P/ASX 200 Index has gained 3.3%. Hardly earth shattering. As ever, it has been a year of volatility, starting in January when Wall Street made its worst start to a calendar year in history.

Brexit. Turnbull. Trump. Yellen. Oil. Gold. Iron ore. Bonds.

Surely 2017 will bring some sense of stability?

Pigs might fly.

Old timers like myself may remember the "good old days" when you looked at your portfolio every few months.

Not so in these days of the 24-hour news cycle. Portfolio viewing can now be a minute-by-minute activity, each look tempting you to dump a loser, add to a winner, or buy today's hot stock.

Oil shares today, anyone?

Speaking of hot stocks, Liquefied Natural Gas Ltd (ASX: LNG) shares are jumping 12% higher to 69 cents.

Last year, LNG was the hottest stock trading on the ASX. Its share price had soared from 30 cents at the start of 2014 to a peak of almost $5 in May 2015.

Gains of 1600% hit the headlines, drawing in the speculators, usually right at the top of the market.

Since its peak, LNG shares have collapsed around 85%. A forgotten hero with no revenues, mounting losses, a dwindling cash position, operating in an industry where it sees LNG demand out-stripping supply in 2021/2022.

Enjoy the 12% jump in the LNG share price today. But don't expect it to last. At best, it's pure speculation. At worst, it's setting money on fire.

I'm constantly amazed by the willingness of otherwise smart people to risk losing all their money trying to get rich quick from investing.

Sure, you might get lucky. You might have bought LNG at 50 cents and sold out at $4.00, bagging yourself a massive winner. Good luck to you.

But most speculations will end in tears.

With share prices and share markets jumping around like a kid on a pogo stick, one of the hardest things in investing is zooming out and focusing on the long-term.

Here at The Motley Fool, we encourage our members to assemble a portfolio of 15 to 30 high quality companies, and to hold them for at least 5 to 10 years.

Not every stock will be a winner.

The portfolio won't rise every year.

But, chosen well, it should make you money, with dividends coming on top.

The rule of 72 tells you roughly how many years it will take to double your money, given a fixed rate of return.

An average annual rate of return of 15% per annum will see you double your money in about 5 years (72 divided by 15 = 4.8 years).

12% doubles in 6 years, 10% in 7 years, 7% in 10 years.

The ASX has historically gained about 10% per annum, over the long-term, including dividends… meaning you'd have doubled your money every 7 years.

With interest rates down at these low levels, it's prudent to expect the next few years to return, on average, something less than 10% per annum.

Say around 7%… meaning your portfolio would double in value over the next 10 years, including dividends.

Unfortunately, when it comes to the finance industry, such messages simply don't sell.

Far better to promise the world, including getting rich quick… ultimately leaving you to pick up the pieces. Property spruikers are past masters.

If you use the simple rule that if it looks too good to be true, it almost certainly is, you'll save yourself a fortune.

Doubling your money every 10 years is very realistic.

Nothing is guaranteed in investing, but I'd be willing to stick my neck out and say a portfolio of well-chosen growth companies would be virtually guaranteed to double in value, including dividends, over the next 10 years.

Of course, it's possible to do it faster. To double your money in seven years, maybe even five years.

You'll need to start by investing in well managed companies with sustainable competitive advantages, and with long growth runways ahead.

I'm not talking your usual suspects — the banks, supermarkets, big miners and Telstra (ASX: TLS). By pure dint of their size, they simply don't have the future growth prospects.

I'm talking about companies like Transurban Group (ASX: TCL). The toll road operator is growing revenues at double-digit rates and trading on a 5% franked dividend yield.

Or a company like Retail Food Group (ASX: RFG). Only yesterday the master franchiser of brands including Gloria Jeans and Brumby's, amongst others, reiterated it expected to growth profits in the 2017 financial year by 20%. The 4.4% fully franked dividend yield is the icing on the cake.

This afternoon, after the market close, our own Scott Phillips will name his top 3 ASX stocks to buy now, exclusively to members of his Motley Fool Share Advisor advisory service.

Scott has publicly recommended numerous stocks that have more than doubled over the past four and a half years, including one that has soared over 700%.

Still, as a stock picker, and someone I consider to be one of the best in the business, he's as modest as they come.

"It's still early days," he'll say, even though a stock he recommended might have already doubled, or more.

Scott's modus operandi is to pick stocks for the long-haul. Stocks like Retail Food Group, up almost 150% from when he first recommended members of Motley Fool Share Advisor buy the shares.

Scott says if you are not prepared to embrace long-term thinking and diversification, then investing isn't for you.

If you've read this far, I'm guessing investing is for you.

You are keen to build a diversified portfolio chock full of high quality, growing companies. And you are keen to double your money over the next 5 to 10 years.

Scott doesn't get every pick right. But he has got an excellent track record, has a very sharp mind, and is an excellent judge of people and companies.

Once a month, he chooses his top 3 "Best Buy Now" ASX stocks. Today is that day. Give Scott a shot by grabbing a 12-month subscription to Motley Fool Share Advisor. Just for you, because you've read this far, I've even gone ahead and knocked 50% off our retail price. But you will have to hurry, because I can't say for sure how long I'll keep offering my 50% off discount.

Click here now to give Scott a shot, and in the process, take a shot yourself at doubling the value of your portfolio over the next five to ten years.

The post The one way you can virtually guarantee to double your money in the stock market appeared first on The Motley Fool Australia.