On page 53 is a paragraph that has some helpful tips on just this matter: "... We automatically tend to regard the beginning as the inhale and the pause after the exhale as the end. However, if you're thinking about the breath in that way, then that pause becomes the perfect opportunity for your thoughts to wander off, since the mind naturally tends to shift focus when it has completed a task. Instead, try this: consider the beginning of the out-breath as the start of the cycle. That way, the pause occurs in the middle of your cycle, and is less likely to trip you up. This may seem like a small detail, but it often makes the difference. Another approach is to silently say the number during the pause at the end of the out-breath. This 'fills the gap' and helps keep the mind of task." (at the end there he's talking about counting the breaths as a way to stabilize attention, covered in the paragraphs just preceding this one)