I picked up this little protective visualization from the book, "Energetic Boundaries" by Cindy Dale: Basically she suggests that if you feel energetically overwhelmed by someone to imagine yourself at the center of 4 strong horses facing away from you, one in each direction. As they walk out, widening the circle away from you, they create a barrier that negative energy cannot penetrate, and that barrier widens as they move away from you (and toward whoever's energy is threatening to overwhelm you).