Keep your guard clean after each use with cold water, a soft toothbrush and hand soap. Let it air dry before returning it to the guard case to avoid any bacterial growth.
Don't use hot water to clean the guard as it can distort the shape/fit of the guard; avoid toothpaste as it can be abrasive; and avoid soaking in alcohol based products as it can reduce the guard's life.
Once a week, give your guard a thorough clean by soaking it in a denture tablet or white vinegar. After soaking for few minutes, remove from the solution and rinse. Be sure to clean your guard case with mild soap occasionally as well.