In this talk Bhante Vimalaramsi gives the full instructions for practicing the breath meditation AS THE BUDDHA taught it. The Buddha did not teach one-pointed concentration which is currently taught by Vipassana teachers, but rather a "knowing of the breath" meditation.
One does not focus on the sensation but on knowing of the breath. Then a step is added, as directed by the Anapanasati Sutta MN 118 and the Sathipatthana Sutta, which is to recognize and then RELAX into any distractions.
Please see the talk for more details. This is very important.