It's easy to rationalize that successful people benefit from a combination of hard work and good timing -- and that if things were different, you too would reap the benefits of all your hard work.

The reality, though, is not as simple as combining a good idea with a lot of hard work and expecting the result to be a profitable business. And while luck can absolutely play a small part, hoping to get lucky won't help you hit it big.

But one thing, one thing shared by every successful entrepreneur, definitely can.

Here's a guest post from Ryan Robinson, an entrepreneur and marketer who teaches people how to create meaningful businesses of their own. (His online courses "Launching a Business While Working" and "Writing a Winning Freelance Proposal" can show you how to start and grow your own business while working a full-time job.)

Here's Ryan:

There is one crucial thing that most, if not all successful entrepreneurs have in common. Before the fame, glamor, and money came, most of them had built up an extreme domain expertise in a particular field. They had become true experts before launching the businesses they've become most well-known for.

Imagine how hard it is to become an expert in a field you know nothing about. Would Bill Gates have succeeded at Microsoft if he had thought of a computer as an oversized typewriter? Could James Dyson have invented the range of vacuum cleaners bearing his name if he had spent his entire life in an office, employing cleaners to tidy up his messes?

While there are undoubtedly exceptions in a world as diverse as ours, let's take a look at six highly successful entrepreneurs who've built powerful self-employed careers for themselves, upon a foundation of great domain expertise. If you're looking to get a jump start on becoming an expert in what matters most to you, pick up this Skill Assessment Sheet today. You'll learn which skills you'll need, and which ones to outsource, in order to become a successful entrepreneur within your field.

Who hasn't heard of Bill Gates? He's one of the most recognized people in the world, and for good reason. Although he has moved his focus elsewhere recently, he gained his initial fame and fortune through founding and building up Microsoft.

Long before Microsoft began, Bill was introduced to computers in the eighth grade, and started spending countless hours learning how to program on various different systems. By time he got around to building computer software and starting Microsoft with Paul Allen at Harvard, Bill had already accrued nearly 10 years of experience writing software.

In the early days of the company, he even personally reviewed every line of Microsoft's code. There would not have been a Microsoft, a Windows, or even a DOS if Bill Gates hadn't tinkered around with code as a teenager.

Another iconic legend of our time is Walt Disney. Yes, Walt himself is now long gone, but his name and legacy will continue long into the future. How is it that the Disney name is still so well known nearly 50 years after his death?

Indeed, not only is his name well known, his evergreen products are as successful today as they were all those years ago. Children today are still entertained by Mickey Mouse, despite the fact that Walt created him in 1928. Before Mickey came along, Walt had been writing comics, telling stories, and creating characters for many years.

The longevity of his success can be attributed to his intense creativity and passion for storytelling, which started at a young age. He pioneered the creation of modern animation and filmmaking, which laid the foundation of the success that all of his businesses would continue to replicate for decades to come.

One of the world's most prolific bloggers is Jon Morrow. He's built up his content-writing knowledge through many years of trial and error, including being the commissioning editor for many years at Copyblogger, one of the most successful content marketing blogs in the world. He's since launched his flagship blog, Boost Blog Traffic.

Morrow now successfully teaches other people how to blog and runs numerous courses, charging fees that most of us can only dream of commanding for one-on-one consultations. Jon is a firm believer that it takes on average six years of hard, consistent work for somebody to become an expert in any given field.

It took him six years to gain the necessary domain expertise to become a master of blogging and he's now on his next six-year stint, learning how to master advertising.

In the world of fiction, there are few names more recognized by both readers and filmgoers than J.K. Rowling. This author and entrepreneur has built up a brand earning her a net worth of approximately $1 billion.

Once again, although there are many reasons for J.K. Rowling's success, the critical underpinning factor is her domain expertise. She began writing at the age of 6 and spent nearly seven years honing the initial idea for Harry Potter before it became the blockbuster it is today.

David Overton is CEO of the Cheesecake Factory. With restaurants located around the world, he has certainly learned a thing or two about growing a global brand. The company also brings in $1.6 billion in annual revenue. David has accomplished what millions dream of: He built a small family business into a hugely successful public company.

It all started back when he recognized the incredible quality of the cheesecakes produced by the family bakery business and sold to restaurants in Los Angeles. He saw an opportunity to focus on this one core product line, became an expert in the niche, and then expanded.

Vacuum cleaners have been used to remove dust from homes for many years. One of the better-known premium brands is the Dyson, named after its inventor, James Dyson.

James gained his expertise from a design background. He studied at the Byam Shaw School of Art for a year and then took courses in interior and furniture design at the Royal College of Art. He invented his first products while in college, and never slowed down.

Like many inventors, his domain expertise is not in the items he sells (he is not a master of vacuuming), or even in engineering. It's in the way he has trained himself over time to recognize household problems and create meaningful solutions. He spent five years researching and developing prototypes for his first Dyson vacuums before they were ready for the world.

What is your area of domain expertise? What are you an expert in?

Now is the time to find your niche and start working toward becoming the absolute best.