Guide to Ānāpānasati

Step Two
Put aside all thoughts, including all worries and plans. You should
reflect on the fact that all conditioned things are impermanent. They
will not follow your wishes but will follow only their own courses. It
is useless to try to grasp them. It is useful to put them aside while you
are meditating.
Whenever you get entangled in thoughts, you should remind yourself
that now is not the time for you to worry; now is the time for you
to keep your mind on only the object of meditation: the breath. If you
happen to remember something very important, and think you must
remember it or think it over, do not do so when you are meditating. If
necessary, you can write the important thing down in a notebook that
you keep beside you, and then not think about it as long as you are
meditating.
If you really want to succeed in ānāpānasati, you have to put aside
all other objects. Some yogis want to develop concentration, yet
cannot give up their attachment for many worldly things. As a result,
their mind becomes restless, always wandering between the breath
and worldly objects. Even though they try hard to calm their mind,
they fail: only because they are unable to give up their attachment for
other objects. Such attachment is a great obstacle to one’s progress in
meditation. So it is very important that you make a strong resolution
to stop all other thoughts while you are meditating.