At the request of Shamash, I am going to do a thread on here about the way I perform Psychic Defense. First of all I think it is important to say just as in any division of Defense, prevention is essential, it is better to know how to protect oneself and prevent an attack then to be fighting one off.
One of the first things to Psychic Defense is know you, know what is you, how you feel, yourself on as many levels as you possibly can. The key here is that it will allow you to know when you are at risk of an invader. The art of Psychic Defense can be compared to Hygeine used to prevent illness.
Think of Grounding exercises (where one goes back to a Base and situated state) as taking a shower, since you are returning to your basic clean state before all the grime of the world go onto you. Ground as regularly as you would take a shower, you can even integrate the two (i.e. Ground whilst taking a shower) that way the connection and the importance can be solidified.
Centering is another common practice in Defense, here is why and what it does: Centering is the art of alignment, going from a negative or positive state to a neutral or "centered" state. The reason one would center is that it gives you a baseline.
So here is the essential difference between Grounding and Centering: Grounding is returning to a Basic state, where all the excess charge can be cycled through the earth leaving only that which is you (just like the dirty water being washed down the drain when you take a shower); Centering is not only finding the baseline that you return to in Grounding, but is also rebalancing yourself so that you don't go stumbling around energetically in the world.
So, Centering helps you since it returns you to a situation where you are in control energetically, since one of the easiest ways for someone to come under attack is when their energy is unstable. Grounding prevents Psychic contamination and long time build-up of Psychic Residue which can linger and cause the systems that are supposed to help you on the energetic level not to work as effectively which makes you vulnerable.
So there you go, this is the basics of Psychic Defense, though Chakras are an excellent tool for Grounding, Centering, and generally interacting with the energetic body (which will also be important for when the time comes for fending/fighting off a Psychic assailant).