I learned the hard way. Years of practice, reading, more reading, more practice, and an occasional video. Something I did though, which I'm grateful for, is that I spent about 2 years shooting in nothing but manual mode. It forced me to consider every factor in the exposure triangle (Shutter Speed, ISO, and Aperture) to get the desired result. I also shot on a lot of film, which made me really think of how I was framing an exposure. After a while, I started remembering rough setting for specific lighting, which really helped a lot, as well. Sometimes, you just don't have the time to fiddle with your exposure. With digital, you can take a picture, immediately look at it and throw it away, if need be. It's cheap to do, and film costs a heck of a lot more to shoot on, hence my desire to actually think about what I was doing. I didn't want to waste rolls of film like I did when I was a kid and not knowing any better. I wanted every shot to count, because every time I pressed that shutter...whatever I shot, I was stuck with.
I'm not going to say that I don't make mistakes and take an occasional bad picture though. We all do. It's inevitable. But I'm grateful for my experiences and how I learned my craft. It led to me opening my own studio, so it wasn't all bad. I love what I do, even when I don't make a profit.