Every skill and habit gets stronger when you practice it. Walking makes you better at walking. Running makes you better at running. Want to be a good reader? Read. Want to be a writer? Write. But if you stop reading for thirty days straight and do other things instead, you'll see what happens to your reading ability. Same thing if you lie around for ten days, then try to take a long walk. Your legs will be weak. The rule is simple: if you want to make something a habit, do it. If you don't want it to become a habit, don't do it. Replace it with something else instead. This applies to your emotions too. When you get angry, realize two things happened. First, you experienced something bad. Second, you strengthened your anger habit. You basically threw fuel on the fire.
Every habit and faculty is maintained and increased by the corresponding actions: the habit of walking by walking, the habit of running by running. If you would be a good reader, read; if a writer, write. But when you shall not have read for thirty days in succession, but have done something else, you will know the consequence. In the same way, if you shall have lain down ten days, get up and attempt to make a long walk, and you will see how your legs are weakened. Generally then if you would make anything a habit, do it; if you would not make it a habit, do not do it, but accustom yourself to do something else in place of it. So it is with respect to the affections of the soul: when you have been angry, you must know that not only has this evil befallen you, but that you have also increased the habit, and in a manner thrown fuel upon fire.