The only entirely safe way to perform oral sex on a man is by using a condom on the penis. Some condoms are flavored for this purpose (chocolate and mint in particular). And the only entirely safe way to perform oral sex on a woman is by using a "dental dam" on the vagina.
A dental dam is a square piece of thin latex that is sometimes used in dental procedures. They are available at dental and medical supply stores. A homemade dam can be made by splitting a condom lengthwise or cutting a piece of plastic wrap (though plastic wrap is not necessarily effective against HIV).
