A well thought out plan about what you need to practice will do the most to improve a person's game.
To add a little to the already great advice by others is how I use all 15 balls to practice a variety of things.
As Bob mentioned, I've just randomly rolled all 15 balls on the table and used nothing but the bridge.
Another way to use all 15 balls is to roll the balls out on the table and use only type of spin to make the balls.
Another is what I call bank, carom, combo. Again, using all 15 balls randomly rolled on the table, the only shots that can be made are a bank, a carom or a combo or any mix of the three. These shots don't some up often in 9 ball, but in 8 ball and 14.1 they can help you keep a run going.
Then there is the rail practice. I put one OB on the table with the CB and see how many times I can hit the OB by going 1,2 or 3 rails.
Also I setup shots that the "standard" stance cannot be used. I'm right handed so everything is from the view point. There are positions on the the table that the balls can be in that the "standard" type of stance can not be used. I couple of examples are the behind the back shot and shooting one handed and as mentioned using the bridge.
I still do one ball drills. One ball from one position to one pocket with one type of english. Then change to using different english from that same spot. Then move to a different spot on the table and repeat.
Here is the best way to improve your stroke. Again using all 15 balls randomly rolled on the table, all shots are to be made at pocket speed only, any ball, in any pocket. Pocket speed being the OB only having enough speed to barely make the pocket. It is harder to stroke slower than you think and it really helps you be aware of how you are stroking when stroking slowly
Plus you will see how little is really need to run 15 balls. You will also increase your range of speeds you can use to make balls. 14.1 rewards being able to slow roll balls.
This is how I practice. The best practice I've had is playing people better than me. This will always point out where I still have weakness's. The only problem now is finding better players. And no, leagues are not for me.
One thing I read by Babe Cranfield about practice is to not make it work. If you do not feel like practicing, don't. More harm than good will be done. If during practicing, you lose the urge to practice stop. Practice when you feel good about being there practicing.
The only problem now is the biggest weakness I have is between by ears right now. Just wish I could find better players outside of having to play in handicapped leagues or tourneys. People where I shoot don't want to play me after besting the house favorites a few times.