Mark Twain was onto something when he wrote this.
Many people struggle to make circular forms when sketching in their subject matter. Here I have illustrated a number of ways to deal with this problem. The center image illustrates an easy way to get more uniform round forms and is as simple as letting gravity do the work. By drawing your lines from top to bottom, left and right (A to B), you are putting less stress on our arm and hand. After all, it is easier to make a downward movement than an upward one and let the law of physics help you out.
It also helps to build your circle within a basic structure that gives you guidelines to work off of, such as square or hexagon shapes. It is much easier to make a box (straight lines) than a circle and will give you a form to work your circular magic within.