I wrote another version of the mantra, for those times when no smiles are present.

Breathing with stillness
Embracing all that comes and goes
Dwelling in this present moment
Knowing it's the only moment

Breathing with impermanence
Unfolding waves of appearances
Dwelling in this present moment
Knowing it's the only moment

Breathing with emptiness
Seeing clearly all that is
Dwelling in this present moment
Knowing it's the only moment

Breathing with non-self
Resting as awareness breathing
Dwelling in this present moment
Knowing it's the only moment

I wrote some alternative versions of that mantra meditation by Thich Nhat Hanh, removing all the references to the self. I was struggling to fully immerse myself into the original mantra, probably because those references conflict with my beliefs of what true meditation is; in which there is no doer, and there is no doing. But these modified versions has been of great use in my practice, so I thought I'd share them.

Internal mantra, spoken with the inner voice. One line with the inbreath, one with the outbreath.

Breathing in with stillness
Breathing out with a smile
Dwelling in this present moment
Knowing it's the only moment

Breathing in with impermanence
Breathing out with a smile
Dwelling in this present moment
Knowing it's the only moment

Breathing in with emptiness
Breathing out with a smile
Dwelling in this present moment
Knowing it's the only moment

Breathing in with non-self
Breathing out with a smile
Dwelling in this present moment
Knowing it's the only moment

Mantra chant, spoken out loud. One line with each outbreath.^[1]

Breathing with stillness
Breathing with a smile
Dwelling in this present moment
Knowing it's the only moment

Breathing with impermanence
Breathing with a smile
Dwelling in this present moment
Knowing it's the only moment

Breathing with emptiness
Breathing with a smile
Dwelling in this present moment
Knowing it's the only moment

Breathing with non-self
Breathing with a smile
Dwelling in this present moment
Knowing it's the only moment

You can play along with repeating only one verse, or saying all in succession. You can also use the Sanskrit terms if you prefer^[2]:
Anitya for impermanence.
Sunyata for emptiness.
Anatman for non-self.

When trying out mantra meditations like these for the first time, I recommend doing some long sessions as well, at least half an hour or more. I find that after a while, the mantra starts speaking itself, needing less and less willful effort to sustain. That is when the conditions are ideal for true meditation; when it's happening all on its own – at which point you can let go of all effort, and just relax in the present moment. Simply resting as awareness.

1. ^{^}

   I prefer to do it this way. I find that when I chant the mantra out loud I'm less likely to get distracted or lost in thought, compared to if I use the inner voice. I use a very soft and effortless voice while chanting it, basically whispering, and I speak slowly so that the words fill the whole outbreath except for a small release at the end.

2. ^{^}

   I'm not sure what the Sanskrit term for 'stillness' would be. When I wrote that part, I was thinking about the boundless stillness that underpins all mental phenomena. Like the stillness – or silence – from which all sounds emerge, and into which all sounds dissipate. I was also thinking about the inherent stillness of awareness; while mental phenomena are in ever constant motion, the space of awareness itself is unmoving. Like clouds moving across the still sky.