Prepare the marinade by mixing the sake, soy sauce, ginger, garlic and dark-brown sugar.
Rinse the salmon fillets under cold water, place in a glass or ceramic dish, and pour the marinade over it. Cover and let marinate in the refrigerator for 1 to 3 hours, spooning the liquid over any exposed parts of the fish once or twice.
Prepare the grill or preheat the broiler to high heat.
Drain the fish and place on foil on grill or a broiler pan. Broil or grill until done (It should be flaky.), being careful not to overcook.
Serve with lemon wedges if you like.
Rice and a steamed vegetable or salad round out the meal nicely.