First thing that came to mind was: Don't believe any promotion or advice that says that if you follow it, "your phone will ring off the hook." I've been burned more than once like that.
More broadly, do your due dilligence. A lot of people talk a good game. Look behind the talk. See if the facts back them up. If you have to ask them for the facts: ("You say you've closed 30 deals in the past 12 months. Prove it.")
And understand that--despite the cautions you hear about investing in the stock market--when dealing with people, past performance is a good indicator of future performance. Someone, or some company, that's treated customers or clients poorly in the past is likely to continue to do so.
And a final one--a very important one: Don't undervalue your services. In many fields (and I'm not being sexist--my wife is adamant on this point--) women seem to have a tendancy to do this more than men. Be very, very reluctant to "offer to do a job for free to prove your value." It doesn't matter if a client asks ("I don't have a big budget right now, but if this deal works I'll have tons of work for you") or whether you think it's your idea. ("I don't have many references or samples, so I'll offer to do it cheap to get some quick references.") Figure out what your time, knowledge, and abilities are worth. Then charge a fair price for them.
Hope that helps.
One is the most dangerous number in business. One supplier, one lead source, one trick pony, one market, one business model, one option (that's actually no option) ... leads to one short career.
Equally important is "I know." When you think or even catch yourself speaking these words it is a INDICATOR your curiosity is waining...followed shortly by inability to be taught.
Finally, surround yourself with others who have secured the wisdom to be on vigilant guard concerning those things upon which their mind and spirit feed. Your spirit will recognize the refreshing cool spring waters they offer.
As far as volunteering, I think you'll find more success volunteering at something other than your profession. If you're a struggling accountant, doing freebies for struggling businesses and non-profits isn't going to get you paying work, but working on a fundraiser for a non-profit just might.
Actually, the one mistake I would caution a new "entrepreneur" to avoid is taking unsolicited advice!
I also have slightly different perspective regarding Mack's advice: "until you have a solid plan of action, pretty much everything you do is a mistake!" I disagree. The best laid plans don't always work, and I could cite numerous examples (especially in engineering and music) of how various people failed their way to success. Don't get me wrong: I'm not saying that plans aren't important. Yet, I believe that determination/desperation is more important than most plans, and is a stronger motivator.
Ultimately, my advice is to do something. Get off the sidelines, and start moving in a direction. It's easier to correct the course of a ship that's moving in the wrong direction, than to get a ship to set sail.
Yeah, that's a big one - the freebie that will lead to more work. Probably never happens. Clients who have real budgets and ongoing work aren't going to "give you a chance" because you're free - they need it done right, and if they think you can do it, they'll pay you for it!
The thing is, everyone should avoid all of the mistakes. Women are, as Don points out, seem more likely to mistake friendliness in the workplace for friendship, and think that the casual acquaintance who offers work but little or no pay is a friend - until you ask when the last time they ever did anything "friendly" together, such as look after your kids when you went to the dentist or something.
I think realistically, you're not an "entrepreneur" without a solid plan of action. Otherwise, you're an unemployed person haphazardly looking for work. I see it in real estate all the time, newbies working on their "marketing," because they don't know what to do to get business. And until you have a solid plan of action, pretty much everything you do is a mistake!