Stay relaxed as this allows you to move faster, with more coordination, power and endurance. This is why professional athletes often say they just have to go out and have fun playing. This way they play relaxed and don’t put too much pressure on themselves. You often see in championship games when a team comes out tight, they don't perform up to their potential. Staying relaxed means using your muscles to their full potential but keep those not in use relaxed. As an example, contracting your bicep bends your arm, while contracting your triceps straightens your arm. Therefore if you want to bend your arm you should contract your bicep fully while keeping your triceps relaxed. Otherwise the muscles will work in opposition to each other. This isn’t something you should actively think about however. Instead if you remain relaxed overall, your body will naturally use the muscles in an efficient manner. Often if you think to much you just create more tension. For overall relaxation work on relaxing your shoulders neck and upper back, many people hold tension in these areas. By relaxing these areas you can often relax the rest of your body as well. Even when not doing martial arts, try to feel when your shoulders are rising up and (if you aren’t doing an action that requires this) try to allow them to just hang naturally without tension. Also look at your hand if fingers are fully extended your usually holding tension in a relaxed state there should be a slight bend. Keep an eye on your elbows also, the points should hang down pointed at the ground (unless an action requires raising them) if you feel them lifting to the sides this shows held tension and also exposes your ribs and centerline to attacks. When practicing try to do so in a smooth flowing manner with relaxed breathing.