Every year at about this time you have to face the facts. Prime cycling season is over. The weather is turning sour and the days are getting shorter. You will still be able to enjoy the occasional ride, but you probably won’t ride enough to improve your performance or endurance. Instead of feeling blue, you can look at the off-season as an opportunity to improve in a different way. Many professional cyclists use their off season to strength train, which can pay off in a big way when prime cycling season comes around again. For cyclists, strength training should be focused at three main areas: legs, core, and upper body.
Legs
If you’re a cyclist, your legs are undoubtedly quite strong already, but a little strength training can help, especially for steep climbs. Squats should be a core part of any cyclists strength training routine. First, learn proper technique by squatting without weights. In time, you will want to add weight, in order to get real strength increases. If you do not know how to squat properly, you will need a trainer who can teach you proper technique. Since squatting is a motion closely related to pedaling, it is one of the best exercises for increasing your power on a bike.
Core
Many experienced cyclists can ride for hours without getting winded and without tiring out their legs. Often the first thing to give out is their core. 
On long rides, they begin to suffer back pain and they cannot continue, even though most of their body isn’t overly tired. To avoid this, you need to focus on core exercises. A good core routine should strengthen the abs and obliques, but cyclists need to put a special focus on their back. Since riding involves being hunched over for hours on end, your back needs to be strong. There are countless ways to design a core workout, but here are some good ideas to start out.
Upper Body
Legs are obviously important for cyclists, and most everyone knows that all athletes benefit from a strong core, but why should cyclists strengthen their upper body? The truth is that cyclists use their arms all the time while they ride. For most of a ride, cyclists carry part of their weight on the handlebars. During hard climbs, cyclists pull on the handlebars to get more force into each pedal-stroke. In the off-season, you should train both pushing and pulling motions with your upper body. Some kind of row is the best pulling exercise for cycling, but if you don’t have access to weights, pull-ups are a good substitute. Push ups are a great way to strengthen your pushing power, and don’t require any special equipment, but their are many other ways to strengthen your arms with weights as well. A plus side to a strengthened upper body is a more balanced muscle structure and less back pain.
You can and should adapt these suggestions to your individual needs. There are many other exercises that can help you reach peak cycling form.
Photos by 182nd Airlift Wing and CCFoodTravel.com via Flickr