Practice
3 Types of pool and all 3 are equally important.
1) Play to play - this is just playing with friends, family or such and having fun
2) Practice - this is structured and purposeful. Working on problem areas and drills.
3) competition - whether league, tournaments, sets with someone equal or better than you
Now what I've found is almost all players spend most of their time doing 1 and some of their time doing 3 but rarely see players doing 2. The players I do see practicing are the ones that tend to be improving.
The 'bring a note book' idea is a great idea. I've done this off and on for years. After anytime you play pool take a few minutes to reflect and write about it. Note the date, time, table, opponent, and then delve deeper. How do you think you played overall? Strengths and weaknesses for that day. Problem shots (draw them to the best you can remember) and include position you were trying to get. I am willing to bet if you kept this information for weeks or months you will notice patterns of similiar problems and weaknesses.
With that done you can practice with purpose. Work on any problem areas you had. Start to structure your practice (however long) to get the most out of it.
An example of a random day of me practicing. Usually starts with me telling eveyone that comes up to me that I don't want to play, I am going to practice by myself. Start with practicing speed. Shoot 15 balls at 1 table length speed, 15 at 2 speed (2 table lengths or lag) and then 15 at 3 speed. Then I shot 10 to 15 straight in shots. Start medium distance and work to longer and longer. Then maybe 10 to 15 cut shots. I may even do 10 to 15 bank shots (short or long rail). By now my arm is loose and I feel ready to play. I usually work some on progressive Stop, Draw or Follow. Follow as example : 5 shots from 1 diamond away - straight in - follow the cue ball 1 diamond. Then 5 from same distance but follow 2 diamonds and then 3 and 4 and 5. Then go through follow but with the object ball 2 diamonds away (do follow 1, 2, 3, 4, and 5 diamonds). Maybe take a short break. Then play a set against the ghost in 9 ball. Break, take ball in hand if if i run out i get a point, if i miss the ghost gets a point. After the set I go back to a couple more drills (i've got hundreds of them).
A big thing to notice is that when you start losing focus you need to take a break, refocus and then come back. Initially your 'focused practice' may only be 30 minutes. It will take work to be able to do focused practice for longer and longer.
Anyone striving to improve should seek out an instructor. They will help with fundamentals and setting up a practice routine and drills to work on. After a week or two the instructor should be able to notice improvement or help with any questions/problems you are having.