Quality Vs Quantity
How many people during their Resistance training do 3 sets of one exercise for a muscle group, then 3 sets of another exercise for the same muscle group, then another 3 sets....resting between sets for a minute or two. They often fail to feel the 'burn' or a 'pump' until their 5th, 6th or even 9th set. You may find that you spend hours pushing, pulling, lifting weights but rarely see any real improvement in your body shape and composition.
There are may different goals when resistance training, strength improvements, specific strength gains to support a sport or activity, recovery from an injury, muscle hypertrophy to name but a few, a lot of people in my experience are looking to improve muscle tone and definition as part of an overall fitness programme. Whatever the goal one thing which is constant is the requirement to overload the muscles, then allow a period of rest and recovery. Every set that you complete should be to failure/overload.
Overload means challenging the muscles to more work than their current level of conditioning allow for. It means performing that last repetition with good form. Good form or technique is everything, you will not get the benefits from the training with poor technique and you may even injure yourself as a result. Good technique also means moving a weight under control, the length of time a set takes you to complete has a bearing on your progression, this is all about time under tension and the optimum time for an exercise to last is 45-50 seconds, in general weights should be moved slowly (moving weights with speed/power is a valid form of training, one which I will discuss another time). Range of motion should not be compromised either, use the full range that the joint allows, without the risk of injury or loss of form.
Intensity in training is a key element to progression, there a lots of ways of making a workout more intense than the usual perform a set, rest, perform a set, rest.....etc. Here is just one for you to try-
Perform a compound exercise (6-12 reps) followed immediately by a single joint exercise (15-20 reps) for the same muscle group.
For example - Dumbbell Chest Press (6-12 reps) > Dumbbell Chest Fly (15-20 reps) Perform 3-4 sets to failure, rest 60 seconds after Fly.
You should choose a weight that you can just about manage the upper end of each range for the first set, it is likely that you wont manage quite as many reps for each subsequent set.
This can be applied to most muscle groups, it is a technique known as 'Post Exhaust'. Try it, I guarantee that if you perform it correctly you will love it.