By holding your own vibration, no lower vibration can communicate with or influence you, and you are upheld by every vibration higher than you.
Having heard this, you may still find it comforting and self-assuring to use a mantra, prayer, or affirmation at the beginning of every intentional mediumship moment. You can call upon all the spirits, gods and goddesses, that you feel good about, and thank them for surrounding you and upholding you as you raise your vibration to communicate with your higher wisdom. You can visualize yourself surrounded and permeated with celestial golden light, as you are enveloped by higher helping energies.
What is important is your intention. You are setting and holding your intention as if you were pushing a button on a remote. You want to receive the channel of everlasting Love and Light. You are pushing this button joyfully with your heart and will.
By remembering that you are Spirit on earth, you open your self up to your greater Self, in whom all love and wisdom resides in oneness.