A proverbial expression meaning that nothing but practice will lead to improvement or success; 'practice, practice, practice' or 'only practice will do'.
上手くなりたければ、練習あるのみだ。
If you want to get better, practice is the only way.
試験勉強も同じで、結局は練習あるのみだと気づいた。
It's the same for exam study — in the end I realized there's nothing but practice.
コーチはいつも『練習あるのみ』と選手を励ます。
The coach always encourages the players with 'practice, practice, practice'.