Soak the fish covered in water with salt and sugar for 10-30 min. About 2 tbsp of salt and the same amount of sugar when using a cup of water.
After soaking, rinse the fish, then rub in the same amount of salt and sugar (like a dry rub). You can add some black pepper too, if you like.
Let the fish sit in a bowl until the oil starts coming out of the fish (30-60 min). Pour out the oil and let sit some more. Keep waiting and pouring out the oil until most of the oil is pulled from the fish.
The amount of fishyness you pull from the fish depends on how long you want to keep pouring off the oil. An hour or two is OK for salmon. It might take overnight in the fridge for fishier seafood.
After extracting the oil, rinse the fish very well. Cook or smoke after that.