Drop-sets are a great technique to use while you lift, but often times people do them incorrectly. The most common mistakes are using too light a weight and dropping too much weight, too many times.  Here’s what you should do: start with a weight that causes failure at the lifting phase at 6-8 reps, at that point drop the weight by 10% and keep going until failure (approximately 4), and repeat until you can’t do more than 2-3.  Try to limit yourself to 2 drops and no more than 15 total reps and decrease the weight by only 10%.  Most people start with a weight they can do for 12 reps then drop it by 50% for another 10 reps – that’s not the way to do it.