Practice makes perfect is the usual statement, but in the case of acting, practice may never make you perfect because this is a very subjective business – perfection is in the eye – and in the ear – of the beholder/listener. But practice can make you better.
Edge Studios has compiled lots of practice scripts for you in a wide variety of voiceover areas. Try them all! Record them all! Review them all! And see if you can find your niche – areas where you shine.
My only critique of the longer form material – such as audio books and documentary narrations – is that there is not enough copy. You need to know that you can read for an hour – or many hours – without getting tired. So if you think you want to practice reading an audio book…then pick up a novel and start reading – and keep reading and reading and reading. That’s the only way you will know if you can do long form material.