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Did you grow up rich? If not, the idea that you'll be wealthy and successful someday can seem almost absurd at times. Let's assume that your parents were good people who worked hard. If they never saw a lot of financial success, why should you believe that you can do it?

The truth is, however, that even if you're starting with little, there are things you can do early in your life and career—in your 20s, mostly—that can make it far more likely you'll be very wealthy by the time you hit 30. Maybe even worth many millions.

Over the last few years I've interviewed a lot of super-wealthy people, first for my book The Intelligent Entrepreneur, and later for Inc.com and elsewhere. Boil down their experiences and there are common things they did earlier in life.

Here are the top 20 pieces of advice I've gathered—the habits and practices that 30-year-old self-made millionaires embraced in their 20s.

1. They develop multiple income sources.

Almost nobody acquires great wealth simply by working for somebody else. In fact, typical millionaires have at least seven different sources of income, and start to acquire them in their 20s. So even if you have a full-time job, it's crucial to develop the attitude that ultimately you are in charge of your income—not your employer.

2. They take action (and sometimes, they fail).

Most people talk, talk, talk, talk—and never do anything. Don't be one of them. Try lots of things. Act. And as a corollary, don't worry about failing. (Surprise: You will fail many times.) As long as you don't do anything ridiculously immoral or illegal, nobody will even remember your failures.

3. They make sure that they own their time.

One thing you'll hear constantly from older wealthy people: You can always make more money, but you can't make more time. As a young person, spend your time doing things that matter to you and that build value. Don't spend it on other people's short-term dramas, and definitely don't slave away for an employer that doesn't value you.

4. They network, a lot. (And they do it both ways.)

What's networking but jargon for "meeting new people." Your network is like a bank, however; you have to make deposits before you can make withdrawals. So make sure to allow yourself to "be networked," too. Even if you're just starting out, chances are you're already in a position where other people would like to meet you.

5. They think strategically.

This is really just a fancy way of saying that wealthy people have usually learned to decide what their goals are, then work backward to figure out what they have to do each year, each week—and even each day—to make it happen. Then they track their progress.

6. They shoot higher than seems reasonable.

All things being equal, it's better to aim at making $20 million and achieve only 10 percent of your goal than it is to aim at making $1 million and achieve it. Because here's a hint: No matter how high you set your goal, once you achieve it, you'll probably think you should have aimed higher anyway.

7. They learn to sell.

Always be eager to show others how the things you have to offer can make their lives better. That's real sales—whether you're selling widgets for work or trying to convince the object of your affection to go on a date with you. Learn to be a good salesperson; that means also being an ethical salesperson.

8. They try to be good friends and colleagues.

Does this one surprise you? It shouldn't. Your reputation is one of your most valuable assets. You want to be the kind of person who makes people think, "Oh, she's great! I'd love to work with her (or for her)." So be the girl who is there when your friends need you, or the guy who people know they can count on. Your friends will remember, and they'll tell others.

9. They invest.

Beyond contributing to their 401(k)s, future millionaires also learn to invest a bit more aggressively. When you're young, you can afford more risk than when you're older (maybe then you'll have kids and a mortgage, for example). Even if you don't have much to invest now, it's still smart to do so on a small scale, even just to develop comfort and make it a habit.

10. They watch their spending.

You don't have to be a cheapskate or miss out on all the fun and adventure life has to offer (in fact, see No. 18 below). But you want to be smart about it. Sure, have a blast--just watch where your money goes, track it, spend deliberately, and always try to get the best deal possible. Speaking of which...

11. They learn to negotiate.

Almost everything in life is negotiable. The wealthiest and most successful people know that the keys to any negotiation are threefold: First, understand what the other side really wants. Next, figure out how to get what you want in a way that gets the other side closer to what they want. But in the end, be willing to walk away if it won't work.

12. They find mentors.

One great thing about the world is that no matter what you want to do, someone out there has probably already done it—or at least part of it. If you can find out who they are and how they got there, it's an incredible advantage. It's an even bigger advantage if you can get them to advise you—and maybe see a little of themselves in you, too.

13. They invest in education.

Don't misread this: It doesn't mean borrow thousands to pursue an advanced degree unless a) you're sure it makes financial sense, and b) you're confident you really want to pursue the career your degree prepares you for. Instead, it means simply becoming a lifelong learner, and not paying more than you have to for your education.

14. They refuse to become slaves.

Lots of people in their 20s quickly submit to things they shouldn't. They accept other people's notions of what a "good" career is, or what a good relationship looks like, or how they should spend their time. Listen to all advice skeptically (including this column!) and embrace the freedom to set your own goals and achieve them.

15. They volunteer to do more.

Volunteer, but do so with three things in mind: First, volunteer to do things that you feel good about. Second, volunteer to do things that will be learning experiences. Third, don't volunteer in a way that makes you somebody's sucker. Example: Don't volunteer at work to do things that won't add to your skill set, won't increase your value, or won't be appreciated.

16. They work their butts off.

Nothing really good comes easily in life, and most self-made multimillionaires will tell you eagerly that their success required lots of hard work. There's one secret that makes this easier, however, which is that hard work doesn't have to mean drudgery. Life is a lot more fun if you spend it working hard at things you really enjoy and value. So choose wisely.

17. They embrace entrepreneurship.

Most self-made multimillionaires are entrepreneurs, so it's smart to learn a bit about becoming an entrepreneur. Learn about business leadership and entrepreneurial finance, sure, but most important: Learn to think like an entrepreneur. I always come back to a modified version of the Harvard Business School definition of the word: the relentless pursuit of opportunity, regardless of how limited your current resources.

18. They have fun and adventures.

Don't let the pursuit of wealth and success block out other opportunities to grow and enjoy life. Travel, write, play music, jump out of airplanes--do the things you'll be talking about for years to come. You'll also find that you get some of your best ideas and solve your thorniest problems when you give your brain a rest and do something else.

19. They believe they can do this.

In a way, this should be the first item on the list. Of course, there are exceptions to every rule. But if you don't succeed at becoming wealthy in your 30s, the largest reason will likely be either that you didn't believe you could when you were in your 20s, or you didn't want it badly enough. The time to overcome that is now.