When I was a teenager, and obsessed with improving, I drilled for hour and hour. I didn't know any fancy / professional drills, but I would shoot the exact same shot for hours. I wouldn't practice shots that were "too hard", but rather shots I would normally make about 60-70% of the time. Through those simple repetitious drills, I could increase that 60-70% to 80-90%. I would go to the pool room that weekend and I would be noticeably better. I was improving in leaps and bounds.
One time, when I was 14 or 15, I got home from school and went right to the basement to practice. I can't recall where my parents were but that particular day I was low on the radar. I practiced all night until the school bus horn told me it was time for school the next morning. I ran out the door without anything to eat, no sleep, and in the same clothes.
As I got older and got some money in my pocket to gamble, and a car to take me to pool rooms, my drills died down to practically nothing. Coincidently enough, my improvement also slowed way down. In fact, I probably haven't improved my skills at all since those days, if I'm playing better, it's only because of experience and playing smarter.
Anyway, I really believe if I would have maintained that drive and stubbornness to keep practicing and drill the way I once did, I would be a much, much stronger player today.