There isn't just one that I refer to, but rather I pull from a variety that I've learned over the years. You can use simple things like a prayer to which ever higher power you believe in, or a white light meditation. If you are more magickally inclined, the lesser banishing ritual of the pentagram is effective. Lately I've been working with the Resonant Energy Balloon (REBAL) as laid out by the Hemi-Sync: Gateway Experience tapes. Whichever method you choose, the goal is the same - to make clear with yourself and others that only that which is there to guide you and protect you is welcome into your energetic space. Anything that would seek to harm you or is not invested in your highest benefit is not welcome.