Wash the salmon fillets thoroughly and place in a pan. In a large bowl, combine soy sauce, olive oil, garlic, ginger, and sugar and mix well. Pour the marinade over the fillets. Refrigerate for 4 to 6 hours.
Preheat a smoker to 250 degrees F with hickory chips.
Place salmon on the smoker. Smoke for 60 to 90 minutes, or until fish flakes nicely.
Serve with freshly squeezed lemon just before serving.
If fish is skinless, cook on foil.