A Codependent is someone with self-esteem issues and poor personal/interpersonal boundaries. Codependency interferes with a person's ability to have healthy, mutual relationships and a healthy sense of self. (Note that this is very common and is readily treatable through therapy, 12-step programs like Al Anon and CoDA, etc.).
A Highly Sensitive Person (HSP) is someone whose nervous system is especially sensitive, such that social stimulation, loud noises, bright lights, coarse fabrics, etc., can cause discomfort or overwhelm. An HSP may or may not be an Empath. This is considered a neutral trait. (For more information, see the work of Elaine N. Aron, Ph.D).
An Empath is, most likely, a Highly Sensitive Person whose energy field naturally merges with others, often for the purpose of helping or relieving the burdens of others. A skilled empath has learned healthy boundaries, while an unskilled empath may have poor boundaries and be quite codependent.
Now that we have that sorted out, let's look at some different avenues available for protecting ourselves from taking on too much of other people's energy. The way I see it, there are three main ways of doing this.
1. Educate yourself on the ways of the empath. Read a great book like Become the Most Important Person in the Room: Your 30-Day Plan for Empath Empowerment by Rose Rosetree. You'll be amazed at all the traps you've fallen into, and how a little knowledge of the terrain can help correct your course. You'll also discover that there are different types of empaths: emotional empaths, intellectual empaths, medical empaths, animal empaths, crystal empaths, etc.
2. Become discriminating in what you expose yourself to. You may not like the word, "avoidance," but until you become an extremely skilled empath, you might have to avoid all sorts of people, places, and things that non-empaths don't worry about. You can probably think of several examples, such as "toxic" co-workers, unattractive environments, or disturbing news stories or movies that you know may overwhelm or drain you.
3. Employ the use of energywork techniques, such as grounding, shielding, clearing and chakra balancing. These practices can be very effective, especially when used along with the above. It should be noted, however, that empaths unconsciously merge with others, so awareness and changes of habit are as necessary for protection as visualizing white light around yourself. (See posts #27-30 from August, 2011 to learn more about simple energywork techniques that are perfect for empaths).