When I refer to protective magic, I'm not talking about astral combat or offensive spellwork. I'm referring to simple things that you can do to ward yourself and your home against harm.
Wards: There is an old tradition that a broom across a doorway will keep harmful spellwork from entering a home. It's a simple matter to lay yours across your doorway whenever you leave. Visualize protective energy surrounding your home as you do. Similar things can be done simply and effectively.
Personal Protection: An old gypsy woman taught me that by braiding a cord out of yarn, charging and blessing it, and wearing it around your waist, you will be protected against most types of magical or psychic harm. Though I've never put it into practice, I don't think she ever took her cord off.
Runes: Runes are also an effective measure of protection. The Rune Algiz should take care of the majority of your protective needs. Wear it around your neck, place it over your doorway, wherever you feel you may need the protection. More potent Runes are best used with a strong background in Rune Lore and shouldn't be used without the proper knowledge.
Keep in mind that it is infinately easier to defend than it is to magically attack. I've talked to Wiccans, Pagans, and Ceremonial Magicians who feel its necessary to have Elemental Guardians and an armada of power animals they can call on at any moment in case they're attacked. Me? I have a feeling that by developing a loving relationship with the spirit world, that most things are headed off before they ever get to me. And that, is the best defense there is.