One way to tame this wild creature in your head is through meditation -- although the paradox is that when you clear your mind for meditation you actually invite the monkey in your mind to play. This is when you are given the opportunity to tame this mental beast by moving beyond thought -- to become aware of a thought rather than thinking a thought. The difference is subtle, but significant. When you are aware of your thoughts, you can let your thoughts rise and float away without letting them pull you in different directions. Being able to concentrate is one of the tools that allows you to slow down your thought process and focus on observing your thoughts.
To develop your concentration, you may want to start by focusing on the breath while you meditate. Whenever your monkey mind starts acting up, observe your thoughts and then return your focus to your breath. Some breathing meditations call on you to focus on the rise and fall of the breath through the abdomen, while others have you concentrate on the sound of the breath. Fire can also be mesmerizing, and focusing on a candle flame is another useful tool for harnessing the mind. Keep the gaze soft and unfocused while observing the color, shape, and movement of the flame, and try not to blink. Close your eyes when you feel the need and continue watching the flame in your head. Chanting, devotional singing, and mantras also still the mind. However you choose to tame the monkey mind, do so with firm kindness. The next time the chattering arises, notice it and then allow it to go away. With practice, your monkey mind will become quiet and so will you.