So here's another 2cents comment on what I've taken from all the (mis)information out there about weight training. First, weight loss is achieved by consuming less calories than you burn in a given day. If you do no aerobic exercize and eat only rice cakes for the next two months you'll loose weight (not recommended). The point is controlling you calorie intake versus the amount of activity you perform will yeild weight loss and shooting for 1-2lbs per week is ideal.
Ok, on to the weight training for cycling. There is substantial evidence suggesting cyclist who increase weight training in the off season will see benefits in performance when spring rolls around (i.e. weight training=good). I do not believe there is any reasonable benefit to weight training more than once a week when you could be out on the road in the spring (i.e. leaves on trees=hit the roads).
As far as exercizes, ALL exercizes should be compound movements requiring more than one joint/muscle group and require the support of your core. The best exercizes are squats (standing barbell), leg extensions (if you feel there is a muscle imbalance in your hammys), bent rows, pullups or lat pull downs, standing shoulder presses, bench press, dips, dead lifts and power cleans.
A typical off season workout for me is a full body routine 2x/wk. This takes about 30 minutes followed by 30minutes running on the treadmill (triathlon training). Squats, Leg Extensions, Tricep Dips, standing dumbell Shoulder Press (I had surgery requiring dumbells but barbell is better), bent rows, core exercizes (lower back and abs). I do 3 sets of each exercize and try for reps 15-12-10 with the last set to failure. The core work is to failure on every exercize and high reps (20-30+). Each set is followed by less than 1 minute recovery and I immediately hit the treadmill for 3-4miles (30minutes).
Lastly, the gym is not your local pub. You go with a training goal, focus, FOCUS, and perform each exercize slowly and correctly. Bring a water bottle so you're not tempted to waste time at the fountain or chatting with your buddy. Weight training in the offseason will pay dividends. But the best return on investment is miles on the bike!! It takes years of training to generate real power so be patient and plan ahead!