Once you reach 30 WPM, you should have the muscle memory of where the keys are down. You should be able to type "The quick brown fox jumps over the lazy dog" without looking at the keyboard. You will most likely be able to continue to improve just by putting in more practice time. Do not be frustrated if you go seemingly weeks without improvement, as progress is not linear. You will plateau at a certain speed, and then all of a sudden feel like you have unlocked speed that you have never reached before. Just make sure that you are typing with high accuracy and good technique.