The Golden Thread or Grounding Cord
One of the earliest grounding techniques I became aware of involved visualizing a golden thread that connected my body to the center of the earth. There are variations on this exercise, but it essentially involved visualizing a pure and strong connection to the planet.
Visualize a Grounding Cord
Like any visualization, you can do it anywhere. People use all sorts of imagery, including rope, thread, umbilical cord, etc. The key is to imagine a thread that drops down out of your spine (or chakra system) and reaches down through the ground all the way to the center of the earth. I sometimes tie the thread around an imagined hot, fiery molten center.
Let Go and Re-Energize
Once connected, you can let go of negativity, fears and anything that you feel is not yours or wish to not be your concern and drop it down into the earth, allowing it to return to its rightful place or person (wherever or whomever that may be).
You can, likewise, allow enriching and vitalizing energy to come up through the center of the earth into your being and regenerating and energizing your body. I first became aware of this exercise when attending various readings that took place at the Berkeley Psychic Institute in Northern California.
Ground to the Planet or to the Galaxy
This grounding technique works very well. My problem with it was that it was limited to the earth, when I personally feel more grounded when I picture the cord going to either the galactic center or the core of reality itself which is essentially nonmaterial and eternal. Still, this works wonders for many.