The benefits of meditating day and night just on death and impermanence are said to be boundless.

One sees that everything that appears is perishable and thereby gains a deep sense of nonattachment to outer objects.

The fire of diligence in the performance of positive actions is set ablaze.

One begins to feel an uncommon and heartfelt fear of the sufferings of cyclic existence.

From recognizing that at the time of death nothing can help one, one gives up the activities of this life.

One uses one’s body, speech, and mind to practice the Dharma, without taking even a moment of ordinary leisure.

One sees how actions mature as results, and this gives rise to determination to be free and disenchantment.

As one knows that the time of death is unpredictable, one does not count on anything.

Numerous virtues that one did not have before are born in one’s mindstream.

One stops believing things are eternal.

One does not have attachment to friends and relations or hatred for enemies.

One is constantly diligent in performing positive actions.

One understands that life is a delusion.

One completes the two accumulations of merit and wisdom, and so on.

The Nirvana Sutra says:

Of all the various kinds of cultivation, reaping the autumn crop is the best;

Of all footprints, that of the elephant is the biggest;

Of all thoughts, that of impermanence and death is the greatest:

It stops all thoughts involved with the three worlds.

The Great Master says:

To obtain the freedoms and advantages is scarcely possible, but even if you do,

There’s no knowing when you’ll die, for death, like lightning’s play, is unpredictable.

With each day, each hour, life is running out,

So don’t defer the practice, Child of my Speech.

Nagarjuna writes:

As if your hair or clothes had just caught fire,

Abandon all to put a stop to this,

And do your best to not be born again:

No greater goal or need is there than that.

And Padampa Sangye:

If first you finish what you have to do, you’ll never get to Dharma;

People of Tingri, while you’re thinking of it, practice straight away.

So reject all the superficial things of this world as though it were so much spit in the dust and practice the perfectly pure, sacred Buddhadharma according to the instructions of the Buddhas and Bodhisattvas. Thinking, “That I might be able to do so, precious teacher, in you I put my trust!” earnestly recite the following prayer:

The perceptions of this life are just temporary conditions, like a dream:

May I be deeply mindful of impermanence and death.

As I pray to you, precious teacher,

Bless me, incomparably kind lord.

Source: Dudjom Rinpoche; Jigdrel Yeshe Dorje. A Torch Lighting the Way to Freedom: Complete Instructions on the Preliminary Practices (Kindle Locations 2104-2136). Shambhala Publications. Kindle Edition.