This is the one I use, both for myself and the kids. How you practice will become how you play, so it's important to make sure you have good form and technique.

There's another, similar saying used in fighting-related disciplines: train how you fight.

Same idea. You're building muscle memory and technique, so make sure your training/practicing matches how you'd do it in a performance (or fight). It's one less thing to have to think about when you're under stress.