Sometimes the weather just doesn't allow for outdoor training in winter. Off-the-bike training can fill in for road riding, and should be part of your regular off-season training routine.
Get in the gym. Lance Armstrong's two main off-season exercises are back extensions and leg-presses. Back extensions build support for the trunk, which supports the power that highly developed cycling muscles generate. Without support, you'll fade when you really need it. Leg presses that develop the quadriceps are the bread-and-butter for cyclists.
Weight gain woes. A common misconception among cyclists is that lifting makes you gain weight. Guess what? You actually stand to lose weight if you're in the gym regularly. You burn a lot of calories working out with weights, far more than if you were sitting at home waiting for the rain to slow down enough to go for a ride.
Gym shy? If a gym isn't for you, run stairs or hills. The uphill efforts simulate weight lifting for your legs, not too mention offering a faster cardiovascular workout than an interval effort on the bike. Beware flat running: Running muscles are not the same as cycling muscles. Start slowly and work up to running distances before counting on running for fitness, lest you make the "oh, I'll just do a few miles" mistake and then limp for a week after going way too far.
Nike made millions (if not billions) with its cross-training shoe philosophy. But Nike didn't come up with that concept; in fact, top-flight athletes have practiced cross training for decades. Doing something other than your chosen sport in the off-season is a good way to stay in shape, and more importantly, avoid burn out.
Get Nordic. Five-time Tour de France winner Miguel Indurain had a V02 max (the baseline rating for cardiovascular capacity) rating of 88, one of the highest ever recorded by a cyclist. Cross-country skier Bjorne Dalhie rated at 93, a full five points higher. In other words, cross-country skiing is great exercise, and it crosses over to cycling very well, with all its big leg-muscle training.
Dive right in. Swimming is also a great cardiovascular workout, if you're an efficient swimmer. It's also a good workout to do on the same day you're in the gym lifting weights for your legs, since it's primarily an upper body workout.
Stretch! Cyclists generally have poor flexibility, which can lead to injury. Yoga or simple at-home routines make a big difference. As with anything, start slowly and build up. Stretching is a workout for your muscles, just a lower-key alternative to riding. While you're stretched out on the carpet, throw in some stomach crunches, which strengthen your trunk.
Head games. The mental aspects of off-season training are important. Set goals, whatever your racing level. Training and racing at 100-percent year-round is next to impossible. Winter is the time for your body and mind to recover and recharge for the season long push. Keep in mind that your off-season work translates directly to real-season results.