Here is an excerpt from my book, The Silent Master: Awakening the Power Within:
5. As you concentrate, you will probably notice that additional thoughts break in sometimes aggressively and you may even find at some point you have abandoned your concept altogether. This is the undisciplined mind in action. It can be really clever in taking you away from your concentration and sending you running down another track.
6. When you find additional thoughts breaking in, or if you find you have abandoned your concentration, do not resist or fight. Have a posture of letting go and quietly, but firmly, returning to the focus of your concentration. As you continue to do this, eventually your mind “learns” not to interrupt your concentration. But you may need considerable practice periods to gain the mastery you desire.
Always state your concentrations in positive terms. Never concentrate on “I am not poor;” say instead, “I am abundantly supplied.” When you state things in the negative, you’re actually concentrating on the negative!