You can start with topo maps, google earth, aerial photographs or if you have access to a small plane...
Look for backwoods swamps, shallow sloughs, ponds, etc. If the water is more than 18" deep during the migration, the birds are only going to be there for loafing. Look for an area that has cover, food, etc. Once you find an area that looks promising, you have to see if it's on public land or if you can get permission to hunt on it. (actually, you might want to start by looking at public land)
If the area is easily accessible, you can bet that someone else is already hunting it. Walk the property and look for shotgun hulls. The more trouble it is to get in to, the more likely it is that you will have the place to yourself.