Following on from my post about the benefits I got from doing a gym session (link here) I came across what I think is an excellent article which I repeat below (with a fee typos removed - yep they must be bad because even I spotted them). The article does not get into too much detail and should give you an insight into what to look to do.
Weight Training for Cyclists
Cycling is well known to be great for building strength in the lower body however cycling does little for upper body strength which can prove a disadvantage for the competitive cyclist. Professional riders have long integrated a weight training element into their training program in order to help them develop core, upper body and lower body muscle strength.
Why do cyclist need to do weight training?
By performing weight training in the pre or early season phase the cyclist can build up the appropriate muscle mass I the gym much quicker than they could on the bike. Once the season starts the cyclist can reduce the strength training significantly, to a level where the muscle mass is merely maintained.
The core strength of a rider should not be under estimated. The abdominal muscles, arms, shoulders and lower back are all crucial muscles that require strength in order to develop an efficient pedalling technique. This is particularly true when talking about sprinting or climbing where riders will often rock the bike in order to help transfer more power to the pedals. In addition a fatigued rider on the flat will often begin to rock sideways, starting at the hips.
Weight training tips
Don't over do it. Lifting extremely heavy weights will significantly increase your risk of injury. Instead mix up you sessions by alternating moderately heavy weights and fewer reps (up to 10 per set) with lighter weights and more reps (up to 30 per set).
Technique is key. Weight training can be very dangerous and lead to injury if not performed correctly. If you are inexperienced make sure you seek advice on technique at your gym from an instructor. Without good form during lifting, you'll be working the wrong muscles which could damage your performance and lead to injury.
Variety is key. Cyclists who weight train are looking for good all round core strength so be sure to not focus too much on a particular set of muscles. Many cyclists simple head straight for the bench press or only perform weighted squats. Do not neglect the slightly less glamorous muscle groups such as the abdominal, back, flexors etc.
It is well know in body building circles that the return phase of a lift (the eccentric phase) is equally, if not more, important than the lifting phase. Be sure not to slam the weights down on this phase and return to your start point in a nice, slow, controlled manner to fully maximise the benefit.
Spending months in the weights room at the gym alone won't make you a great cyclist. Think of your weight training as a quick way to build muscle mass, especially during the winter or at time when you are unable to ride. You will still need to work on your on bike strength to turn this mass into more cycling specific form/strength.
The actual article can be found here: http://cycling.dingdangdo.com/weight-training