Combination of a lot of things;
1. As Chuck said above, a misspent youth, I literally grew up in a pool hall, spending up to 4-8 hours a day 6-7 days a week playing.
2. Playing stiff competition, while you need to spend some time playing alone to work on things I spent most of my time playing better players for a lot of years.
3. Gambling, while some will frown on this nothing will harden you like playing better players for your hard earned $. It will also combined with #2 above teach you to be able to honestly appraise your speed & others speed & therefore how to match up, either you'll learn or go bust a lot. 4. Developing a solid regular pre shot routine, learning to evaluate the table during this routine & make your decisions, then most importantly that moment of disconnect where the evaluations stop, you shut off your conscious mind and get down on the ball & allow your subconscious & muscle memory to execute what you've envisioned during your pre shot routine.
5. Lastly & what I feel is most important & can be worked on away from the table, your mental game. Learning to truly envision what it is you desire to execute. The part of your brain that will get your body to execute shots & position play is driven by pictures in your mind, not by words. Picturing the shot being executed, seeing yourself staying down on the ball with a proper follow through on your stroke, the ball being pocketed and the cue ball moving in line for your next shot. I find this to be the most crucial element. If you don't "see it" or envision it, it doesn't happen. I find this to be the most neglected part of most people's games. Developing this is also what helps you shut everything else out, the rail birds, the $, the pressure that people don't realize doesn't really exist anywhere but in their own minds & they actually put on themselves, the only place fear & pressure exists is in your mind & feeling it or not feeling it is a choice YOU make. Once you've put in a certain amount of time and developed solid fundamentals this mental aspect is the factor that will bring a quantum leap in your game.
Why am I the Colonel? Because I always get the chicken