Oral sex has long been held as something that proper people just don't do, in fact even today there is somewhat of a stigma attached to the act, with people sometimes thinking that someone is “whorish” for performing oral sex. Entertainingly enough, however, despite the fact that there are people out there that would condemn someone for this act nearly every sexually active person does it, even those who would denounce the act.
Small social science lesson aside it's time to get down to the real meat and potatoes so to speak. Being that men and women (thankfully) have different equipment there are two different main strategies for performing oral sex, and an unlimited number of specific twists that can make the experience unique. As funny as it sounds, the only real way to get better at oral sex is to practice, practice, practice, I'm not saying be a whore about it, stick to one partner if possible, but great fellatio masters are made, not born, so pay your dues, an learn from you experience.
Lets start with performing oral sex on a man: There are two main pointers that must be held above all else when discussing oral sex on a man, watch the teeth, and use your hands. If you have particularly good oral skills you can likely hold your teeth in such a way that there is no issue with them scraping the sensitive skin, however for the novice, or under practiced, it is suggested that you roll your lips back over your teeth providing a sort of cushion, this can be slightly uncomfortable at first, but hey, better safe then sorry. Next are the hands. This is a penis, not a Popsicle; it's ok to touch it, even encouraged. While you are sucking it is unlikely that you will be able o deep throat the entire thing all of the time (breathing is important too) so while you are focusing you attention on the head of the penis use your hands to massage the testicles and the penis shaft. Trust me, not enough people do this, and your partner will appreciate the extra attention. Once you have graduated beyond the basic penis sucking try your luck at deep throating, don't be discouraged if you gag your first few times, it's only natural, just keep up the practice, and don't give up. Another handy tip is using the forbidden teeth. True it's painful when your teeth accidentally rough up an unsuspecting penis, but when done with purpose (and gently) it can be very erotic.
Performing oral sex on a woman is physically easier then performing it on a man, but requires a bit more subtly. Like swimming in a lake, you can't just dive right in without surveying the area and checking for rocks. Keeping things interesting is one of the most important aspects. Unlike men, who are often just happy that someone is down there, women expect a little more effort, and can bore easily. For beginners, unless you know your partner likes it rough, do not bite hard; gentle nibbling is erotic, hard biting is assault. Keep in mind that your tongue is no long term replacement for a penis (or dildo). That is to say, do not focus on only penetrating the vagina with your tongue, move it around a bit, shake things up.
Most importantly, and this is for both sexes, pay attention to your partners reactions. If they like what you are doing, do it more, if they don't, try something else.