Did you get the license before you started picking up rental properties, or after?
I was an accidental landlord for a few years - moved, rented out old house for a while, also got divorced and rented out rooms in my own place to pay the bills, etc. During those few years, I got good at the accounting and tax filing, and learned a few things about leases and best practices, but didn't graduate to any kind of purposeful, systematic approach that would make real money. Right around the time that I started kicking myself to get serious and save some real money so I could do it right, applying all the lessons learned, I met a couple of guys who were older and more disciplined but much less experienced with real estate, and who had been talking about rentals for a year or so but didn't know where to start. We figured out our skills and interests complemented each other well, and while talking about a partnership, I mentioned I had been toying with getting licensed. Between their agreeing to split the licensing costs three ways, and knowing I'd have enough business from our own buys to at least cover the $2K or so in annual costs, it was the trigger I needed. I've been profitable from the start, and the bigger the portfolio gets, the faster we'll buy.
Also, did you have to get sponsored by an existing broker for your license and work under them for awhile to make it "official"?
That's pretty much the standard everywhere. You pass the course(s), tests, background check, etc, and a broker still has to vouch for you. Every salesperson who isn't a broker has to hang their license with a broker. Here you can take another class and a test to get licensed as a broker after two years (IIRC), at which point you can leave the company and start your own, or you can be an associate broker with the same company.
Personally, I'm not interested in being a broker and may never be. I came in at a 70% split and am moving to 80% now that I'm fully fledged. In exchange for that cut, the broker handles a fairly extensive set of legal requirements that I am not at all interested in having to manage, including keeping me updated on new laws and professional standards; provides administrative support to ensure all my deals are properly documented and archived for audits by the state; provides technical support, including client interaction tools; gets me a good deal on a group buy for Errors and Omissions (E&O) insurance; reminds me of continuing education requirements and offers free or cheap classes to fulfill them; and generally does more than enough to earn their cut.
Oh, and they throw great parties with free food at least once a month. ;)