Excessive physical activity just before a round should be avoided. I don't allow my players to swim on days that they have a tournament round - this can also cause blood shot eyes. Playing video games causes eye strain and fatigue and should be avoided on tournament days. Players should avoid eating a big meal just before a round. However, a player should never be hungry or thirsty during a game.
Psychology plays a role in how a player performs.
A cap can help keep light out of your eyes and block the ability of your opponent to see your eyes. It is much easier for a coach to find their players when wearing a team shirt and cap. When you make your move don't have your hand hover near a piece. This shows a lack of confidence in your play, you may be falsely accused of having touched the piece and your own hand blocks your view. Keep your hands away from the board until you know what your move is.