I was doing something very similar last night. If you take a piece of cardboard or such that is big enough to block light to your print, when the exposure is complete for the foreground, woosh the cardboard in so it covers it and, starting from the horizon, gradually cover up the whole sky and then pull it back down to the horizon so the top gets more exposure than the bottom of the sky. If your horizon isn't level you might need to get a little more creative with shapes.
I was also printing the foreground with a different contrast filter than the sky, that might help you too.