Investing is simple, but not easy. And anyone can be successful at it, as long as they're willing to make some basic, but crucial changes in their habits. These are among the most important insights I've learned during 10 years at The Motley Fool.

My personal story is unremarkable, but possibly instructive for many middle-aged folks. When I arrived at The Motley Fool in August 2004, I was 41 years old, and had been a teacher for most of my career. If I were to grade my personal finances back then, I'd give them a "gentleman's D+." Now, after a decade of learning, investing, and being more sensible, I can give the family finances a solid "B". There was nothing earth-shattering about that dramatic transformation, but it did require some meaningful changes.

It isn't rocket science, but...
Successful investing is simpler than most people think. The fundamental aim is to spend less than you earn, thereby creating a surplus that can be invested in reasonably priced assets for the long term. And the long time frame is especially key, since that allows you to benefit from the magic of compounding. Additional funds – inheritances, bonuses, windfalls, etc. – are, of course, part of any "surplus" that can be invested.

The key here is to methodically put money to work in the market, year in and year out. Over time, your pot of capital will grow. For Warren Buffett the process is like a snowball: "The important thing is finding wet snow and a really long hill."

Yes, you might be thinking, but if growing wealth is so straightforward, then why are we all so terrible at it? A recent study revealed that 36% of Americans have absolutely nothing saved up for retirement. Additional research shows that 60% of American workers have less than $25,000 saved for retirement.

Sadly, saving isn't the only thing we're bad at. Those Americans who are able to put some money aside often don't make the best investment decisions. According to Richard Bernstein of Richard Bernstein Advisors, the performance of the average investor over the past 20 years is "shockingly poor." According to Bernstein, ordinary investors have underperformed almost every asset class by choosing to buy high and sell low.

So investing might be simple, but it's not easy. Let's unpack this statement a bit more, so we can learn how ordinary investors might succeed at this important endeavor.

The art of money getting
P.T. Barnum once said that the key to wealth "consists simply in spending less than we earn." Yet, he added "more cases of failure arise from mistakes on this point than almost any other."

For many of us, spending less than we earn is a lot harder than all of the personal finance maxims suggest. Let's start with the income side of the ledger. When I was a teacher, my salary was so modest that it barely paid the bills. My investing plan at that time more or less involved hoping for a miracle. I suspect a lot Americans find themselves in similar circumstances nowadays. And just recognizing you need more income doesn't necessarily mean you can get it, especially in a tough economy.

Fortunately, things are more promising on the "expense" side of the equation. Most of us probably spend too much and can make meaningful cuts to our monthly budget, though it requires effort and sacrifice. For example, my family recently cut our monthly expenses significantly by refinancing our house. We've also been making a conscious effort to eat out less frequently. Working to get the "out-go" (as Barnum called it) lower than your income is critical to eventually generating a surplus to invest with.

Remember too that any time you can make investing automatic – like participating in a 401(k) plan, for example – then you've made the job even easier. A small decision to maximize a financial benefit offered by your employer can actually become a game-changing wealth creator for you over the long haul. For me, just signing up for the Fool's generous 401(k) plan on day one has been critical to improving the family finances.

What do I do with my new pot of capital?
So, how do you achieve reasonable investing returns once you are able to generate a surplus of funds? Warren Buffett laid out a very simple plan for most investors in his recent shareholder letter:

What I advise here is essentially identical to certain instructions I've laid out in my will...Put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P 500 index fund (I suggest Vanguard's.) I believe the trust's long-term results from this policy will be superior to those attained by most investors – whether pension funds, institutions or individuals – who employ high-fee managers.

Buffett's plan will work beautifully for the vast majority of Americans. While I personally choose to invest in individual companies, I have the luxury of working side-by-side with folks who think about stocks all day long, and I enjoy learning about the individual companies I decide to invest in. For the non-professional, who may not have a lot of time to study companies, Buffett's strategy is the best one you'll find. The more I think about all of this stuff, the greater appreciation I have for the simple S&P 500 index fund.

Get rich reading books
Once you finally have an investing strategy in place, you'll need to develop your temperament, so you don't buy and sell your assets at all the wrong times. Nobel Prize winner Dr. Daniel Kahneman, author of Thinking, Fast and Slow, teaches that all of us are overconfident and that we are "often blind to our own blindness." For that reason, we need mental fortitude in order to avoid excessive trading and market timing based on half-baked ideas picked up from the financial media.

Reading great books will help you with your temperament. Investing and business books are excellent choices, but so are texts on history, philosophy, and whatever else you are interested in. That's the great thing about investing – every subject is relevant in some way or another.

So summing up, here's what I've learned:

  • Spend less than you make.
  • Put that surplus to work in reasonably priced assets over a long time frame.
  • Develop your temperament so you don't make poor investment decisions.

It was a very good year
2004 was an incredible year for me. I obtained an amazing job at The Motley Fool, and my lovely daughter Sophie was born.

In addition to all of the investing lessons, I've also learned about the importance of gratitude. I've received many blessings over the past 10 years, and I'm very grateful for all of them.