This is a discussion of grounding and centering that Mama Rose collected from users of ISCA BBS, I have borrowed it with her permission until I can get around to synthesizing a document on grounding and centering. My nature combines mostly earth and fire. However, it isn't always apropriate to light a candle, or pull out my zippo to produce a flame to ground and focus with.
Grounding involves connection with the earth, often to cleanse one's personal energy.
To my knowledge this is almost always done to earth. For me, the perfect image to channel my thoughts into calm and peace and let excess energy drain away is that of sinking into a pool of water. Mentally, I look up at the blue sky and crisp white clouds above me, take in the living green colors of sunlight shining through the leaves all around the pool, and let myself exhale. I drift down from the warm surface water, my eyes still open to watch the silver bubbles rolling upward, and sink into quietness, through the thermocline into the cool bottom layers where I hang in dim stillness. This may sound a bit odd to some of you, but when I DO ground, which isn't very often, I actually tune in mainly to Air, believe it or not.