Start with 1 cup salt and 1/2 cup sugar per gallon of water. This is the basic brine. But remember, the brine carries with it the flavors that are in the brine and so can be the vehicle to flavor the meat with savory or sweet herbs/spices. Just remember to not add any acidic ingredients to the brine, such as citrus, as it will inhibit the absorption of the brine into the bird. For my tastes, I would add the following for each gallon of water:
1 tbs. Coarse grind black pepper
1 tsp. granulated garlic
2 bay leaves
1/tsp. each Sage and Thyme
1 sprig fresh Rosemarry, bruised
You could also add herbs such as cloves, basil, marjoram, tumeric, etc. If you want a more smokey bird, with a bit of sweetness, you might try the following. To the basic brine, add:
1/2 cup pure Maple Syrup (B-grade dark for a richer maple flavor)
or 1/4 cup clover honey
This will work very well with the salty brine, serving to ballance the flavors and will also give the skin a gorgeous color when cooked.
Others will shortly arrive with more suggestions. I wish you the best of luck with your birds. Just make sure to take them away from the heat when an accurate meat thermometer reads 150 degrees F., and let rest for 15 to 20 minutes before carving.
Seeeeeya; Goodweed of the North