You get stronger by asking a little more over time.
What we do: Every workout gives you a rep range. When you hit the top of it with good form, you go up in weight next time. This is called double progression, and you track it in your log.
Why: Gradually increasing the demand on your muscles is the core principle of strength training in the American College of Sports Medicine's position on resistance training. Double progression is my way of making it simple to follow. [1]