Combine salt, sugar and pepper and herbs. Spread one-half of the salt, sugar and herb mixture over the bottom of a glass, plastic or ceramic baking dish. Place the salmon side on top of the mixture. Distribute the remaining mixture evenly over the top of the salmon. Arrange lemon and lime slices over mixture. Cover with a double layer of foil. Put a board on top and about 4 pounds of cans or bricks on the board. Leave in the refrigerator for 3 days, turning the salmon and basting with the juices twice daily. After 3 days, pour off the juices and pat the fish gently with paper towels.