If you want to achieve overall purification of your nature, then japa can be most effective if you do it in a systematic way, step by step. On the first day repeat Aum or "Supreme" or whatever mantra your Master has given you five hundred times. The next day repeat it six hundred times; the day after that, seven hundred; and so on, until you reach twelve hundred at the end of one week. Then begin descending each day until you reach five hundred again. In this way you can climb up the tree and climb down the tree.
Please continue this exercise, week by week, for a month. Whether you want to change your name or not, the world will give you a new name: purity. While you are doing japa, if you make a mistake and lose track of the number, no harm. Just continue with some likely number. The purpose of counting is to separate your consciousness from other things. When you count, you will not be thinking of someone else or something else. While you are counting, you should try to enter into the world of silence which is deep inside the mantra. Then you will not have to count at all. Your consciousness will be focused on what you are repeating and you will begin to feel that you are meditating only on the inner significance of the mantra.
In most cases it is best to chant a mantra aloud. But after doing this for a few minutes, if you can feel that there is somebody inside-your inner being-who is repeating the mantra on your behalf, then you do not have to chant out loud. In the silence of your heart your inner being will do japa on your behalf.
Japa should be done in the morning or during the day, not just before going to bed. If japa is done when the body is tired and wants to enter into the world of sleep, the mind will just become agitated and lose its one-pointed concentration. You will only be working the mind mechanically, and you will derive no benefit. If japa is not done sincerely and soulfully, it is useless. So it should be done only one hundred, two hundred or at most three hundred times before going to bed. If you meditate before going to bed, you will invoke peace, light and bliss, but if you do japa five hundred to twelve hundred times before going to bed, you will invoke power and energy, and you will not be able to sleep.
Often when you complete your japa, you will hear the mantra being repeated inside your heart. Your mouth is not saying it, but your inner being has started repeating the mantra spontaneously.