Thich Nhat Hanh:
Counting is an excellent method for beginners. Breathing in, count “one”. Breathing out, count “two”. Continue up to ten and then start again. If at any time you forget where you are, begin again with “one”. The method of counting helps us refrain from dwelling on troublesome thoughts: instead we concentrate on our breathing and the number. When we have developed some control over our thinking, counting may become burdensome, and we can abandon it and just follow the breath itself. This is called “Following”.
Well-known commentaries (on the Sutra on Full Awareness of Breathing), … teach that while we breathe, we should be aware of our nostrils, the place where the air enters and leaves the body. Just as when we cut a log we keep our eyes on the place where the saw touches the log (rather than looking at the teeth of the saw), we pay attention to the nostrils, and not to the air as it enters the body. … If the practitioner focuses his mind at the tip of his nose and is aware of the first moment of contact of air at its place of entry into the body … gradually his rough, uneven breathing will become gentle and subtle, and then all discrimination will disappear.
I will add that “Counting” is not only useful for beginners, but also when I am sitting in the waiting room at the dentist’s or standing in line at the grocery store. I also find that touching the tip of my index finger to the tip of my thumb on “One” and then the middle finger on “Two” and so on to the little finger on each hand, gives me a count of eight and helps me keep my focus, for example when walking.