A lot of people are not getting the most bang for their buck out of resistance training. They swing the weights to get them moving or perform the rep quickly, using momentum to cheat and gravity to lower the weight back down. A person can throw around more weight by cheating, AND they run a higher risk of injury because they are not actually strong enough to control the weight. This is part of the reason why pushups and other bodyweight moves are so hard, it's difficult to cheat. Understanding a few basics about how your muscle works will help you bang that buck.
When you contract your muscle(as in curl UP in a bicep curl) you are making a CONCENTRIC movement--your muscle fibers are shortening. When you let the weight go back to the starting point you are making an ECCENTRIC movement--when done PROPERLY your muscle fibers are shortening at the same time they are lengthening to release the weight.
On the bike you use a lot of eccentric muscle work to stay upright, absorb shock, remain relaxed and balanced and resist the g-forces of landing. If you focus only on the concentric/contraction phase you are losing out on gaining strength and stamina for your eccentric muscle efforts. IE half-ass. Half-ass BAAAAAAD!
So when you are lifting FIRST learn proper form, this will prevent swinging weight to cheat and also make you actually target the muscle/s you are focusing on. Second, slow down that lift and release. Count 1-2 up, 1-2 down. Focus on resisting the weight being pulled down by gravity. Don't just release the weight down, or relax your muscle to let it go, RESIST(this *is* called resistance training!).
Next, occasionally, --->after you have a good base of strength training<---use the count 1-2/1-2-3-4. This will really work the eccentric phase. Keep in mind that this phase is more stressful and difficult on your muscle because your muscle is doing two things at once (shortening/lengthening). You are engaging more resources, a good thing, but also more opportunity for injury if not respected. For this same reason do this with low weight/high reps. You will avoid injury and boost endurance. The other day I used the 1-2/1-2-3-4 counting throughout my upper body work. It was definitely much harder to complete my reps--at this point I am ready to up my weights, 3x12 is easy to complete, using the longer counting made it difficult to complete the reps and I got nice and sore.
Now for the kicker--after all that, riding gives you the best SPORT SPECIFIC eccentric workout. Seat time rocks again!
Here is some more reading(bonehead type):