I agree that watching the pros or any good player very closely, particularly technique, helped me more than other instructional sources.
Another thing that helped my game jump, surprisingly, was taking breaks. I played/practiced/gambled etc. every day for several years, and during that period it never felt like I had my stroke settled and could trust it, some days it was on others it was off, sometimes I felt like I would practice until I unlearned how to make a friggin ball. I was forced to take a long break, and when I got back on the table, although it took me a little bit to regain my ability to shoot, I found that I was distilling my technique into its most essential elements. Practicing so much earlier surely helped but it also led me to obsess over minute, relatively unimportant things that would eventually make me loose sight of the important elements of the game. Nowadays if I take a break every once in a while, usually from 1-6 weeks, I find that when I get on the table I get my game in gear faster and I'm more comfortable with it. I don't think I could be where I'm at without all the practice but I would have never discovered the benefits of a break without being forced into it. Before I'd worry that if I took a few days off my game would tank (it did), now I don't sweat it and look forward to playing with enjoyment when I want to.
Psychology studies have shown that people work with information best when they are given a chance to learn it gradually and sleep on it or otherwise have a break period before they are required to use it. This is not true for everything, a break won't help you memorize numbers unless you do so repeatedly. However, this is the case for so-called procedural knowledge, knowledge of how to perform a series of motor actions where you might not necessarily be able to verbalize what you are doing easily (i.e. tying your shoe). I knew that procedural knowledge benefited from a so-called consolidation period, but fear of having my game drop off kept me from taking breaks that probably would have only helped in the end. Breaks also keep you hungry to play the game, so you don't end up playing bored and developing bad habits.