21st Century Mage (in the library) has a bunch of options for a daily practice to prepare yourself to invoke your HGA. This is one of them. It's very short, so can quite easily be added to a daily ritual stack. I've only been doing it a week, but it has felt powerful and I'm keen to see how things progress with it. The rest of this post is verbatim from the book (I recorded it as a guided meditation for myself, to be used for a short while till it's memorised)
Shin is the Hebrew letter for the sound "sh." In Qabalistic symbolism, this letter signifies the spirit element. The Shin of Shin Operation is a ritual formula based on the Hermetic Order of the Golden Dawn's Neophyte ritual. The purpose of the Shin of Shin Ritual is to obtain the Knowledge and Conversation of the Holy Guardian Angel.
For simplicity, the Shin of Shin Operation can be reduced to a series of mental operations that result in the same effect. Magic need not be complicated in order to work.
I highly recommend that you begin this practice by performing a banishing ritual and then a self-purification and consecration.
Performing the Shin of Shin Operation
Sit in a position in which you can remain comfortably still.
Close your eyes and imagine that your body is a black egg.* It should be a deep flat black like absolute emptiness.
Imagine that you were floating above a black sea.
On your left is a giant pillar made of cloud. On your right a giant pillar of fire.
The two pillars descent deep into the darkness below you and rise up into the heavens infinitely far above you.
The right pillar represents the mercy and love of the universe, and the left pillar represents the severity and cruelty of the universe.
To equilibrate these opposites, imagine a point of divine white brilliance above you, shining down on your egg.
This is the descent of unity and wholeness, the descent of your [Holy Guardian Angel / Holy Diamon / Higher Genius / preferred term].
Pause and feel the two pillars and the light above.
Between you and this holiness, imagine a flashing sword that is like lightning.
Aspire towards the light.
With all your power, push all of your personal energy up through your spine.
(You are still a black egg, however.)
Push all of your personal energy up through your spine towards the point of light, keeping your attention on the flashing sword.
The more you push towards the light, the closer the flashing sword comes to you.
As you push with all your might toward the light, imagine the flashing sword descending upon you and striking you on the back of your neck, the shock and pain of which destroys your conscious personality.
Pause and feel your emptiness.
The two pillars and the white light are still there.
The holy light then sends a ray of light down upon you, spiralling around your egg three times, and your egg turns grey from this influx of light.
Pause and feel the small amount of light within you.
The holy light descends upon you again, spiralling around your egg three times, until your egg is nearly white.
Aspire again towards the light with all your power.
Push all of your personal energy up through your spine towards this light, keeping your attention on the flashing solid that is still before you.
The more you push towards the light, the closer the flashing sword comes to you.
As you push with all of your might towards the light, imagine the flashing sword descending upon you and striking you on the back of your neck, destroying you again.
This time a giant pyramid of flame appears in the midst of the divine light above you.
You feel the light filling you and the fire growing brighter.
Withdraw for a moment and show yourself that the two pillars still remain at your sides.
Then, with all of your might, aspire to and invoke the light and the fire that is above you, and allow all to disintegrate into this infinite boundless light.
* This is the Akashic Egg, which is the Indian Tattwa symbol that corresponds to the Western mystical concept of the spirit element. In many cultures, the egg is an archetypal symbol of the spiritual body and the universe.
It will probably take a bit of time for this operation to have an impact on you. The advantage of this technique is that it operates in pure symbols and has no dogmatic implications.
It is a very beautiful and powerful meditation. If this formula appeals to you, but you would like to do it as a ritual, simply act out the components within a magical circle.

Shin is the Hebrew letter for the sound "sh." In Qabalistic symbolism, this letter signifies the spirit element. The Shin of Shin Operation is a ritual formula based on the Hermetic Order of the Golden Dawn's Neophyte ritual. The purpose of the Shin of Shin Ritual is to obtain the Knowledge and Conversation of the Holy Guardian Angel.
For simplicity, the Shin of Shin Operation can be reduced to a series of mental operations that result in the same effect. Magic need not be complicated in order to work.
I highly recommend that you begin this practice by performing a banishing ritual and then a self-purification and consecration.
Performing the Shin of Shin Operation
Sit in a position in which you can remain comfortably still.
Close your eyes and imagine that your body is a black egg.* It should be a deep flat black like absolute emptiness.
Imagine that you were floating above a black sea.
On your left is a giant pillar made of cloud. On your right a giant pillar of fire.
The two pillars descent deep into the darkness below you and rise up into the heavens infinitely far above you.
The right pillar represents the mercy and love of the universe, and the left pillar represents the severity and cruelty of the universe.
To equilibrate these opposites, imagine a point of divine white brilliance above you, shining down on your egg.
This is the descent of unity and wholeness, the descent of your [Holy Guardian Angel / Holy Diamon / Higher Genius / preferred term].
Pause and feel the two pillars and the light above.
Between you and this holiness, imagine a flashing sword that is like lightning.
Aspire towards the light.
With all your power, push all of your personal energy up through your spine.
(You are still a black egg, however.)
Push all of your personal energy up through your spine towards the point of light, keeping your attention on the flashing sword.
The more you push towards the light, the closer the flashing sword comes to you.
As you push with all your might toward the light, imagine the flashing sword descending upon you and striking you on the back of your neck, the shock and pain of which destroys your conscious personality.
Pause and feel your emptiness.
The two pillars and the white light are still there.
The holy light then sends a ray of light down upon you, spiralling around your egg three times, and your egg turns grey from this influx of light.
Pause and feel the small amount of light within you.
The holy light descends upon you again, spiralling around your egg three times, until your egg is nearly white.
Aspire again towards the light with all your power.
Push all of your personal energy up through your spine towards this light, keeping your attention on the flashing solid that is still before you.
The more you push towards the light, the closer the flashing sword comes to you.
As you push with all of your might towards the light, imagine the flashing sword descending upon you and striking you on the back of your neck, destroying you again.
This time a giant pyramid of flame appears in the midst of the divine light above you.
You feel the light filling you and the fire growing brighter.
Withdraw for a moment and show yourself that the two pillars still remain at your sides.
Then, with all of your might, aspire to and invoke the light and the fire that is above you, and allow all to disintegrate into this infinite boundless light.
* This is the Akashic Egg, which is the Indian Tattwa symbol that corresponds to the Western mystical concept of the spirit element. In many cultures, the egg is an archetypal symbol of the spiritual body and the universe.
It will probably take a bit of time for this operation to have an impact on you. The advantage of this technique is that it operates in pure symbols and has no dogmatic implications.
It is a very beautiful and powerful meditation. If this formula appeals to you, but you would like to do it as a ritual, simply act out the components within a magical circle.