You cannot prepare enough for anything.
Never just run through a study because you happen to be familiar with it, but use it to see what you can get from it on this new day which has been granted you.
You can sightread better if you know your scales and arpeggios.
Control of vibrato helps your musical expression.
Taking drugs to overcome nerves is the thin edge of the wedge going in there.
Running through things because you are familiar with them, breeds routine and this is the seed of boredom.