I have been working out completely wrong for much of my life.
First, understand muscle growth: you get stronger when your muscles break down and rebuild themselves. (This is why eating a healthy amount of protein is important. It helps your body rebuild stronger muscles.)
So, the first step in working out is breaking down your muscles.
However, for many years I’ve prescribed myself a certain number of repetitions, i.e. fifteen push-ups, and done those in sets over and over. This works, but the problem is that I’m not pushing my body to complete exhaustion.
When you push a muscle to exhaustion—your maximum effort until you fail or your form breaks down—your body tends to come back stronger. It adjusts for the strain you put it under, and a few days later, will be ready for just a tiny bit more. Slowly but surely, you gain strength.
If you only ever do enough reps to feel like you’ve done something but aren’t outputting the maximum, it’s hard to get stronger, because your body doesn’t have to adapt to anything.
(Something I learned from The Dip by Seth Godin, a great book that applies to way more than working out.)