The most valuable piece of advice that anyone gave me was a high school friend who was a more advanced player than I was. He said, "If you want to get better, play at least an hour a day, every day." At the time we would jam on the weekend for six hours, and I might practice some chords once or twice after I had done my homework. I said, "I'm going to try that," and I really felt my playing rise. It gave me confidence that this works, and that encouraged me.
The other thing is that it has to be playing. While I was very committed to developing technique, when I was doing eight hours a day it was like two hours of technique, it was two hours of theory, it was two hours of improvisation, and then it was two hours of "free swim," where I could either do more technique or I'd work on some funny sounds or whatever. Some of that felt like work, but a lot of it was really fun. The third thing is you get better when you play with other people, and you get much better when you play live. The way to get better is to bounce music off other people, and have your ears open, and to play live.