Friction causes the skin to thicken and develop a callus. Calluses are commonly found over bony prominences because there is less padding -- muscle and fat -- between the bone and skin. A "writer's bump" is a callus that forms typically at the last joint of the middle finger of the hand you write with. It develops over time from the pressure of pens and other writing tools. This bump is harmless, however it can be uncomfortable and does affect the way the finger looks. Although this bump might not disappear completely, there are several ways to reduce its size.
Take care of the skin around your writer's bump to decrease discomfort and swelling. This area often becomes red and sore after long periods of writing. Use a pea-sized amount of lotion and apply it to this area. Gently massage it to decrease pain and improve circulation. This can also decrease the swelling which will reduce the size of the bump.
Use a pen that requires less pressure when writing. Pens with gel ink flow easily and require less pressure to be applied to the paper. This will reduce the pinch force required to hold the pen, which lessens the pressure on this area of the finger. Over time this will improve the way the finger feels and looks.
Use a rubber gripper and place it on your pen or pencil. There are many different kinds available on the market -- some are a standard cylindrical shape while others have indentations where each fingertip should be placed. There are also adaptive writing implements on the market that completely change the technique used for writing and eliminate pressure where the writer's bump has formed.