Side hustles are extremely popular. An estimated 44 million people in the U.S. have a side hustle; that's over six times the number of workers who hold multiple jobs. The process is easy; it only takes a few hours to set up your own small business.

Making a side hustle actually pay off, both professionally and personally? That's a lot harder.

Over the last year I talked to scores of people who started successful side hustles to find out what works, what doesn't work, and the lessons they've learned.

Here are their 10 commandments for side-hustle success:

1. Consistently crush your full-time job.

Side hustles are fun. Side hustles are distracting. It's tempting to think about, dwell on, and even occasionally work on your side hustle when you're at your full-time job.

Don't. One, it's just wrong. And two, you aren't that stealthy. People will notice -- and no matter what your side-hustle dreams, losing your full-time income is the last thing you can afford.

When you start a side hustle, your goal is to be great at two things: Your full-time job and your side hustle. 

So even before you start your side hustle, focus on being a superstar at your full-time job. Work as hard and efficiently as possible. Get more done than anyone else, if only so you can leave on time without regret, and without raising concerns about your performance and dedication.

2. Never take on side-hustle debt.

Many business ventures require spending money before making money. That's why some small businesses take years to turn a profit. A huge percentage of startups fail because they never turn a profit -- much less pay back their investment. 

How can you avoid that? Start a side hustle you can fund through savings -- or better yet, that you don't need to fund. Provide a service that only requires the tools you already have. Sell products you either make or can procure by consignment. Prove to yourself that there is a market -- and that you can serve that market -- before you take on any debt.

I know: It's tempting to think, But if I just had this... and this... and this... then I'd have a real business

The only real business is a business that turns a profit -- and it's a lot harder to turn a profit when you're paying off debt you didn't need to incur.

If you can't find a way to start your side hustle without going into debt, find a different idea.

3. Don't use a side hustle as an excuse.

Imagine metalworking is your hobby. You've always wanted a larger forge. So you start a side hustle. And you buy the bigger forge.

But you don't need it. Not yet. You just want it.

Plenty of failed side-hustlers admit they started their business as a way to rationalize the purchase of something they had always wanted to own. A nicer car "because I will need to make a great impression on clients." A bigger workshop "because I will need room for all the woodworking equipment I'll need."  

If you just want something -- and there's nothing wrong with that -- don't use starting a side hustle as an excuse to buy it.

4. Don't spend money the customer won't see.

You might have a cool office at your full-time job. You might have cool amenities. 

Don't think your side hustle should.

Before you spend money, always ask yourself one question: "Does (this) touch the customer?" If it doesn't, don't buy it.

Spend what money you have where it makes a real difference to your customers -- because without customers, you don't have a side hustle.

5. Only spend money on actual efficiency.

It's tempting to think ahead, to forecast need, then spend money based on those forecasts.

Like needing more supplies before you actually have demand. Or needing more equipment before you actually have demand.

Or needing greater efficiency before you need to be that efficient. Sure, buying a certain tool could make you X times faster at performing a certain task, but if you currently don't have enough customers who will pay you to perform that task beyond your current capacity, don't buy it.

Allow yourself to be inefficient until you have enough work to make greater efficiency truly matter. 

6. Always follow a strict side-hustle schedule.

When your "normal" workday ends, your side hustle work day is just beginning.

Decide how many hours you think you can spend a day on your side hustle. Then add 25 to 50 percent to that number. If you're thinking two hours, make it three or four.

Then commit to that schedule. Write it down, and if your schedule says you will work from 6 p.m. to 9 p.m. every evening, and from 9 a.m. to 3 p.m. on weekends, work those hours.

See the schedule you create for your startup the same way you see your schedule for your current job -- as non-negotiable.

Then work that schedule.

Otherwise you won't see any progress, you'll quickly get discouraged, and you will never have given yourself a fighting chance to succeed.

7. Dream big, but focus small.

Almost every side-hustler dreams of finding an enabling customer, that one big customer that will let you bypass the "hustle" and truly launch a business. 

How many actually find an enabling customer? Out of over one hundred people I talked to, one. (And that was an accident; the customer approached her before she had thought about starting a side hustle.)  

Instead, do what Dharmesh Shah recommends: Rather than finding a way to make a million dollars, find a way to serve a million customers. Start small. Prospect where you have a reasonable chance of success. 

Along the way you'll learn. You'll build your skills. You'll build a customer base. Later, you can leverage that customer base -- and everything you've learned -- to successfully hunt bigger game.

8. Only do what generates revenue.

Sure, you might need to spend a little time on admin and infrastructure. But not much. You don't need fancy spreadsheets. You don't need comprehensive reports. You don't need a catchy brand or a mission statement.

What do you need? You need work -- and you need to do the work so you can get paid. Successful side-hustlers focus on two things: Selling and working.

Take me. I only make money when I'm writing or speaking. Anything else I do, however "important" it may seem, is time taken away from what really matters: Generating revenue.

It may be true that when you do what you love the money will follow, but only if what you love doing actually generates revenue. If it doesn't pay, put it away.

Successful entrepreneurs eventually spend more time working on their business than in their business. Later, that might be true for you as well, but for now, a successful side hustle requires working in the business -- because that's the only time you actually make money.

9. Always default to action.

Making plans is great.

But stuff always happens. 

Most people who start a side hustle don't make it past their first three action items before adapting to reality. (I started a company assuming I'd provide book design services to publishers; I ended up ghostwriting books instead.)

Spend a little time planning. Then spend a lot more time doing. If you're unsure, do something -- and then react appropriately.

It's easy to think and plan and evaluate yourself out of ever starting a side hustle.

Remember, it's not life and death: It's a side hustle. See starting a side business as the grand experiment it is. Never forget that the fun is in the doing -- not the thinking.

10. See your side hustle as "me time."

When you choose your side hustle, pick something you want to do. Pick something you want to achieve. Pick something you want to be, and actively work toward it.

Not only will you enjoy the sense of accomplishment that comes with progressing toward a goal -- even if that goal is doing something purely for fun -- but you'll also feel better about yourself and your life. 

In short, see your side hustle time as "me time." Because it is -- it's time you spend making the most of your life. See it as time you spend that will leave you feeling fulfilled.

Sure, other people might be chilling with Netflix. And that is one form of "me time." 

But so is a side hustle -- because when you choose the right side hustle, and you give it your all, that means you're making the most of every hour you have.

Which is the perfect definition of "me time."

And is the best way to truly live.