Pool & billiards is a sport, so like any other sport you must train your body and mind to develop the skills required to excel. Here are some tips to help you improve your game, based on skill level:
Before you develop bad habits, the best thing you should do is work on your fundamentals. Having a consistent routine and a reliable stroke is crucial for playing pool & billiards, and it will make the sport way more fun!
As you're learning how to pocket and position the cue ball, it can be beneficial to understand the physics of the interactions. After every missed shot or position, try to figure out why the outcome was different than your plan.
If you already have good fundamentals, then you probably want to improve your consistency with pocketing and cue ball control, which will allow you to run more racks. The best thing you can do here is drill, drill, drill. This type of practice can often be boring, so that's why I created Bullseye Billiards - which gives you a target for every shot.
Instructional aids such as books and videos can provide a ton of information at a low cost. I recommend getting some of these, but make sure you follow the advice and keep up the practice! You can't get better just by reading a book or watching a YouTube video.
If you are really serious about improving, then you should definitely get an instructor or attend a pool school. There are many, many areas of the sport that you could be having issues with, so a qualified expert will be able to give you personal guidance to accelerate your game to the next level.
Most of the work for an advanced player will be on strategy and the mental game. You should put yourself into high pressure situations (but not too high!), which will provide motivation for you to concentrate and really bring out your skills. Strategy can be developed through competitive play and analyzing the decisions of top players.
Ironically, the mental game becomes much easier if you have great fundamentals and practice a lot. Repeating the same motions over and over will cause them to become more automatic with time, and that is truly the best thing you can do for your game.
No Magic Bullet
There's no magic bullet or quick fix that will suddenly make you better! Sport psychology tells us it takes a few weeks to build a new habit, and even longer to make it consistent. It takes time to see improvement, but if you follow the advice above, then you will start to see more runouts and higher rankings.
With that said, sometimes you will play above your normal abilities (which is an awesome feeling!), but try to have realistic expectations about your average game. Sometimes you will not play great, so hopefully that motivates you to get better. Everyone makes mistakes, but practice will help you make fewer mistakes.
For additional advice on getting better at pool & billiards, check out my Resources for Learning article. I also offer a number of instructional aids on the Bullseye Billiards website that I have found very useful.