Playing in the Present
Learn to play your game in the present. What does this mean? When you are playing, it is easy to start thinking about mistakes (I like to call them Learning Experiences) you have made in the past or fears about what might happen in the future. When you do this, you are not playing "in the present" at that time.
Why is this important to understand? In order to play "in the zone" your mind and body must be working in harmony, playing the game "in the present," focusing on the play at hand, not the past or the outcome.
You see, if you are playing in the present, there is no pressure on you. Pressure exists when you dwell on mistakes of the past, or fears about the future. Think about this for second. If you are truly focused on the play at hand, and only concerned with doing your best at that moment, you will have no pressure on you. This means your muscles will be loose and relaxed. Your mind will be focused. You will be at your best!
FOCUS ON THE PROCESS, NOT THE OUTCOME
This takes practice. You need to work at it. Your habitual thinking can start without you even noticing. Start paying attention to your thoughts during the game when the play is out of your zone. If you are thinking about the past or future, you need to take a deep breath or two and clear your mind. Use a buzzword such as "Focus" or "Clear" and continue to breathe deeply while focusing on the play at that moment. You will get better at clearing out this past and future thinking over time.