To get more juice out of a lemon, break down the lemon by putting some pressure on it before cutting it. Jennifer Semon Rostal writes:

Before cutting them, put them on your counter and roll them back and forth, applying heavy pressure with the heel of your hand. This will help break down the lemon and make them easier to juice! Use a tool like a citrus reamer to get out more juice than you could squeezing with your bare hands

Tips and Tricks - Lemons | A Self-Confessed Foodie, Photo by Bobbi Bowers.