While plastic wrap may be great for protecting your food in the refrigerator, it has questionable value in preventing the transmission of disease during oral sex. Of course, any kind of barrier is better than nothing at all.
Some people use latex dental dams as protective barriers.
A dental dam is a thin, square piece of latex that can be placed over your partner's vagina or rectum during oral sex. After using a dental dam once, throw it away. Although using dental dams can't guarantee you'll stay safe, you can use them to reduce your risks of getting a sexually transmitted disease and make sex safer. To buy dental dams, look for dental supply stores in the yellow pages.