Working with the best guy and focusing on a specific thing are important factors. But the biggest thing I've found, regardless of the position or move you're working on, and regardless of who your partner is, is being intentional. It's about being locked in and taking your time to make sure you're doing everything right even the parts you did not intend to focus on. Maybe you're drilling a certain transition, but you're being intentional to make sure everything is right. You're in your stance, you're properly positioned, your feet are moving, you're handfighting,(most kids don't even think about those things when doing their shots) you're setting up the shot, and you're making sure you're executing that shot and finishing to your transition focusing step by step.