It is clear that the risk of transmitting HIV is much, MUCH lower for unprotected oral sex than for unprotected anal or vaginal sex, and that the risk is MUCH lower for the person being sucked or licked than for the person doing the sucking or licking. For the person doing the sucking or licking, the risk of transmission is lower if your gums (and lips/mouth/throat) are healthy, if you don't let men come in your mouth, and if you don't perform cunnilingus on a woman while she is menstruating.
Some people recommend NOT flossing or brushing your teeth for an hour before giving unprotected oral sex (use Cool Mint Listerine or some other anti-bacterial mouthwash if you're concerned about bad breath or just want to freshen up), and others recommend quickly looking over the genitals you're about to go down on for obvious signs of contagious sexually transmitted infections (including genital warts, which can on rare occasion be transmitted from genitals to mouth). If your policy for performing unprotected cock-sucking is to not let your partner come in your mouth and he does so anyway, it's better to immediately spit than to either wait or swallow, and it may help (especially for bacterial infections) to then go use an anti-bacterial or peroxide mouthwash. Pre-cum can contain HIV, and although not letting men come in your mouth SIGNIFICANTLY reduces your already low risk to even lower levels, if you are concerned about becoming infected via pre-cum while performing fellatio you have two risk-reduction options: not taking the head of his penis in your mouth or using barriers for oral sex.
If after getting all the facts you decide that your personal safety standards include barriers for oral sex, then you'll need to use latex condoms (without Nonoxynol-9) for cock-sucking, and either regular saran wrap or one of those "Glyde" dams for eating vag (for this, put a little water-based lube on your partner's side of the barrier to increase the sensation transmitted to her). The same barrier techniques used for cunnilingus can also be used for rimming (licking someone's asshole), where they should be considered essential if the person doing the licking isn't immunized against hepatitis A or if the person being licked may have a bacterial infection.