Cast a streamer tight to timber or the bank, let it sink, and retrieve it slowly with long, pausing strips. Cod often follow a fly a long way before they eat, so keep the fly moving to the rod tip and stay ready. Use a strong leader (15 lb or heavier) and strike by pulling the line firmly, not by lifting the rod, so the hook sets in a big jaw.
On a warm evening, try a large hopper or other surface fly along a grassy bank. Fish the near bank first, then the far one.