If you've spent any time in the metaphysical or Pagan community, you've probably heard people use the term "shielding." Shielding is a way of protecting yourself from psychic, mental, or magical attack -- it's basically a way of creating an energy barrier around yourself that other people can't penetrate. Think about the Star Trek series, when the Enterprise would activate its cloaking device. The magical shield works much the same way.
Remember that energy exercise you did when you learned how to center? When you ground, you push excess energy out of your body. When you shield, you envelope yourself with it. Focus on your energy core, and expand it outward so that it covers your entire body. Ideally, you'll want it to extend past the surface of your body, so that it's almost as though you're walking around in a bubble. People who can see auras often recognize shielding in others -- attend a metaphysical event and you may hear someone say, "Your aura is huge!" It's because people who attend these events have often learned how to shield themselves from those would drain them of energy.
When you're forming your energy shield, it's a good idea to visualize the surface of it as being reflective. This not only protects you from negative influences and energy, it can repel them back to the original sender. Another way of looking at it is like the tinted windows on your car -- it's just enough to let in sunlight and good things, but keeps all the negative away.
If you're someone who is often affected by the emotions of others -- if certain people make you feel drained and exhausted by their very presence -- then you need to practice shielding techniques. Also, be sure to read about Magical Self Defense.