Professionally one of the best things I've learned is how to say "no". When I was just starting out I wanted to work as much as possible so I said yes to everything that came along. The low point came one year when I did my taxes and realized that, although I had gigged quite a bit, I only grossed about a thousand dollars doing it. I had said yes to everything and allowed myself to be bullied into lowering my rates to practically nothing. So I decided to take some time off from playing and just focus on teaching. I said no for the next 3 years.
When I started performing again I was more specific about the fee I expected. There were some who understood and others who didn't. Before my hiatus, I was hired to play only one song while a guy proposed marriage to his girlfriend. I showed up and played the song and the guy paid me ten bucks. To him that was a lot for only one song, but it was a little over an hour out of my day. Those days were officially over. It occurred to me that every product and service has a predetermined price affixed to it. You don't haggle with a dentist or at the grocery store. If someone was unwilling to pay my price then I wouldn't play.
I also had to learn to say yes at the appropriate times. I couldn't just dig in and say no all of the time. When I was asked by a wedding to coordinator if I was willing to play a certain pop song I said "yes". She was thrilled. The other musicians had all said "no" and some were rather rude about it. That was the beginning of a nice professional relationship that I cherish to this day.
Now when a bride or groom request a special song I try to accommodate them. This has inadvertently caused me to reinvent myself from "classical" guitarist to "wedding" guitarist. I play instrumental versions of pop songs on solo guitar. I now play more gigs than ever before and get paid much better than before. A side benefit is that this style of guitar has enjoyed a huge rise in popularity in recent years so many of my students want to play this way too.
Another unexpected benefit is that I really enjoy doing this. I would never be able to play such diverse music in any other venue or format. I enjoy (usually) the challenge of arranging and learning the music. To go from a solid classical wedding one day to a pop music wedding the next day and a mixture the following weekend is something that I've become pretty good at. And that all started with saying "yes".