In guidelines for Japa part 4, Mahanidhi Swami talks about the obstacle of the wandering mind and the ways that we can avoid the wandering mind during our Japa.

Control the Mind

"Pull it back when it wanders. Chanters Beware!!! Yoga sastras say that tamo-guna and sleep usually attack a sadhaka after he has practiced one hour of meditation. In other words, after sitting peacefully and chanting japa for one hour, one should be watchful and check his energy and attention levels. If you feel drowsy or dull headed, then chant loudly to shake off tamo-guna; go outside for a japa walk; drink some cold water and splash some on your face; do sirsasana (headstand) for 2 minutes and 10 rounds of pranayama; or a few stretching exercises. Refreshed and invigorated, sit before Tulasi-devi again and renew your japa meditation."

Sleepiness can be a very great obstacle in our Japa and Maharaja points out some good ways to counteract this. We need to be aware of ourselves and how our energy is in our body (without this becoming a distraction). If we detect that our levels are starting to dwindle, then we need to take action to overcome that....drinking cold water and splashing some on our face may help. Walking outside can also be very helpful, fresh air can keep us from falling into drowsiness. We are all individual, so whatever it takes, we need to take action when this occurs.

"Cut the tendency to recall events from yesterday, last week or the year before. The bird of the mind flies on two wings: the past and the future. Cut off the wings. Be here now: Hear, chant and remember-love, serve and surrender! “Forget the past that sleeps; and never the future dream at all. Deal with times that are with you, and progress you shall call.” - from Sarnagati.
Don’t plan today’s duties. If the mind persistenly dwells on the “shopping list” or “things to do today”, then stop briefly, write down the points, and chant japa with a tranquil mind."

Here the Maharaja talks about a very important point....to avoid listening to the mind recalling events from the past or thinking about the future....normally what we have to do in our day. Ignore all these thoughts and just listen to the mantra, don't listen to your mind.

From The Art of Chanting Hare Krsna by Mahanidhi Swami