Maintaining or even gaining strength is the absolute best way to make sure that you're not losing muscle. If you keep pushing big weights, it'll force the body to keep its muscle since it will see it as necessary for survival. If you reduce the amount of weight you're lifting, the body will "assume" that you don't require as much strength and that it's okay to lower your muscle mass. Why? Because muscle uses a ton of calories every day and the body will see it as expendable.