This is a practice from my book Leap Before You Look.
Lie down on your back
Under a cloudless sky.
Open yourself to the nature of infinity.
Let yourself move out infinitely in any direction
And be soberly present with the unavoidable fact
That as far as you travel,
You are still only halfway there.
There are no limits.
You cannot think about infinity.
It will blow your mind.
You can only become one with the infinite.
Look into the sky without blinking.
Let the sky enter you, so there is no inner and outer remaining.
Then close your eyes and stare into the inner limitless sky.
This simple practice, to look into the open, cloudless sky, has been used in every tradition in every age. I was introduced to it in the Tibetan refugee community of Bodh Nath in Katmandhu by the great Dzogchen teacher Choki Nyima Rimpoche. Once it is pointed out to you, it becomes so obvious--it was there all along. Many of us get involved in spiritual practices and teachings, searching for who we really are, and the answer is right there above our heads all along: you only have to look up into the vastness of the sky. This practice is great for modern humanity, as we have grown so used to a man-made world. Everything has been modified; everything has our fingerprints all over it, except the sky. The vastness of the sky cannot be touched, cannot be modified; it remains the last outpost of absolute innocence.
The sky is such a great teacher, a great mirror, because it is not a thing. It is a presence. It contains all things, but it cannot be contained. Things have a beginning and an end in time, things have limits in space. But the sky has no creation date, it can never be destroyed, and it has no physical limits. There is a word for it, but there is no thing to which the word points, only spaciousness.
The sky can teach you about your own true nature. Consciousness is also not a thing. It also contains all of your experience, although it is not an experience. It, too, was never created, and has no limits. You also have a name, but if you follow that name back to where it points, you will find nothing there.
Imagine a vast empty space, extending equally in all directions. Now imagine a door in that space, without any wall around it, just a door in a door frame. There is space on one side of the door, and there is space on the other side of the door. Step through the door, and you step out of space and into space. How many spaces are there; how many infinities?
Staring into the sky is just like that. Look up into the sky, and there is infinite space. Look back into yourself, and there is infinite space. That which you call "me" is just a door in infinity, with infinite space behind it and before it and all around it. The infinite space of the sky and the infinite space of who you truly are are not two. "Space" is a word for Oneness.
You can discover 72 practices like this in Leap Before You Look.