After I started riding 6 years ago, I have found that the best results came from a combination of strength training (gradually harder throughout the off season), and endurance training using preferably cross country skiing or running. To keep the bicycle muscles in shape, I do a couple of spinning classes a week (usually in combination with the strength training). Cross country skiing has especially a great effect on your endurance because your are using more muscles of your body and the ones close to your heart. And, you get to go outside! It takes some time to get the technique just right, but with practice...

The imortant thing is to focus on quality (not quantity) and to keep yourself motivated during off season by varying your training methods. Try yoga, pilates, core training, etc that you normally don't have time for during the season. Also, plan your training and keep a journal - it's the only way you learn what works and what doesn't. Or, if you can afford it, get a personal trainer

Good luck!