The worst kind of practice is when you just throw the balls out on the table and shoot without caring what happens. If you just practice for fun, you will end up improving a little, but not much.
The best kind of practice is to set goals for yourself and try to meet them. When i used to teach, I would give my students this drill: Practice 8-ball by taking 2 balls (one of your group and the 8-ball). Throw them out on the table and let them stay where ever they land. Take cue ball in hand and run both balls. Keep this up until you can do this consistently 8 or 9 times out of 10. Then move up to three balls (2 from your group + 8 ball). Depending on how good you are, this may be easy and you can probably skip some of this and jump right to 4 or 5 balls and the 8-ball. If you do not run out, try to reposition the balls the same way and try again, and again until you see how to get out. If you are not getting out because you are just missing a ball, you should work on your shot making. If you are not getting out because you are running the balls in the wrong order, this drill will help you recognize certain patterns and you will eventually start running the balls in the correct order. If you have a layout that you don't know how to run, ask someone else who shoots better and see what they think.
This drill can be applied to 9 ball as well. Start with the 8 and 9 ball, then move up to the 7-8-9 ball, etc. Eventually you will be running the balls better and more consistently.
If you find yourself playing good in practice but poorly under pressure, like in a tournament or a money match, you can practice for that also. You need to play competitions with yourself. For example, I would play myself a race to 11. If I broke, made a ball on the break, and ran out, I would mark one up for myself. If I didn't do all of that, I would mark one up for my opponent. This is a very hard drill and the best I ever did was lose 11-8 (usually the score was 11-(3, 4, or 5)). You need to find some kind of game that you can play by yourself. The idea is to put pressure on yourself. If you practice just for fun, you miss out on that pressure, which is why a lot of players play better for fun and poorly for real.
Good luck.
Andy