I've thought about giving how-tos on here for a while now, but I've demurred time and time again. For some reason, I've always felt that it's presumptuous and possibly even conceited to write one. However, having never felt that way about reading how-tos on anything in the past, I've decided to get over this worry, and give a couple pointers on oral sex.
I will state this many more times, but the thing to stress above all else is that everyone is different, and there is nothing that is guaranteed to work with any one person. Above all, communication is key. Wanting feedback means you're a good, concerned lover, not a bad one. Oral sex, like every other aspect of sex, is something that you do not to someone, but with someone. Now, onto the good stuff. Ladies first.
A common worry that women have about getting head is that they won't smell or taste good. An easy way to combat that is to go down on her in the shower, or directly after a shower. However, in most cases, if a woman has decent hygiene, she has no reason to worry. But if she's worried, she's not going to be letting herself have as good of a time as she might otherwise. This is why it can be a really nice idea to let her know if you like the way she smells or tastes. Obviously if you don't actually like it, you don't need to lie about it. But a very large number of gals and guys do like the way most women smell and taste, and letting the woman know that you're enjoying yourself can help her relax and enjoy herself. This can extend itself to making it clear in other ways that you're having fun. You don't need to turn into a porn star overnight, but making a couple noises if you feel like it can be a welcome addition. Also, before or after you actually do it, just outright telling her that you enjoy it can help make her feel a lot more comfortable about it. Again, no reason to lie, but it's nice to hear if it's true.

Besides, why wouldn't you enjoy it? She'll look like this afterwards. Guaranteed.
Women, on the whole, are a lot slower to warm up when it comes to sex, and are a lot more patient about that process. Guys, you don't need to dive right in and go for the clit. In fact, you don't even need to dive in and go for the vagina. Hang out with other places, and slowly start narrowing your focus. You want her to be pretty damn excited about it before you even touch her vagina, much less her clitoris. For some women I know, you pretty much can't tease them for too long. This certainly isn't the case for everyone, but guys, err on the side of too much buildup. Probably way, way too much.
Another difference between men and women for sexual pleasure is that usually when a man gets really excited, or something feels great, or he's getting close, it means it'll be even better if you do it harder and faster. This is rarely the case for women. Most often, when a woman starts ramping up for an orgasm, you shouldn't ramp up physically to try to accomodate. You're already doing something that feels great to her; you don't need to change a thing. Similarly, you don't need to change techniques too often, as sometimes it can break the flow. Maybe it's just me, but whenever I'm doing pretty much anything with a girl, I sometimes feel like I'm being boring unless I change things up pretty regularly. Then I remember all of the times I've been in the opposite situation, and very rarely get bored with what the girl is doing. It took me a long time to realize that I can do one thing for a fairly long time if she's enjoying it, and I won't be boring for it. Maybe I get antsy more quickly than most people, though. If you feel like you already spend ten minutes at a time without switching anything up, don't take this advice as me telling you that you should try keeping it up for twenty (unless she asks you to).
The clitoris is analogous to the head of the penis. It has the scientific name of glans, just like the head. However, the clitoris has many, many more nerve endings (around 8,000, twice that of the male glans), and it's much, much smaller. With this in mind, do not treat the clitoris like you would the head of a penis. This more than most things depends on the woman. Some women are sensitive enough that they don't even like their clitoris to be touched directly until they are very aroused, if at all. Others you can be a little rougher with, but don't go crazy. If it's not too intense for the girl, you can try lifting back the clitoral hood* and licking the glans directly. This is something you may want to ask about before doing, or at the very least be very attentive to her response. It can be too intense for a lot of girls, or be just the thing to send another into an orgasm.
If you start to run out of ideas, get tired, or just want to keep things interesting, feel free to use your hands, too. Some women really appreciate the switchoff, or sometimes the combination. Trying to use your hands at the same time can be a little difficult, but it's worth experimenting with. Some girls just like having the fullness of having something in their vagina at the same time, while others like the sensation of both areas being pleased at the same time. It's not essential for most women, but it can be fun to try out. Also much easier logistically is bringing in a toy. Either you or the girl can manage the toy while you're doing your own thing with your mouth.
Women more than men tend to not focus as intently on the orgasm. This is something men could take a note from. You don't need to spend every second worrying about how you're going to get her to come. Everything you're doing down there feels good, and that can be enough. To that end, you don't need to worry about keeping up a certain momentum, or taking breaks, especially early on. Before my door gets beaten down, however, I'm not saying you shouldn't make a valiant attempt to give your lady friend an orgasm, or that all women are just fine with being aroused and stimulated without a climax. But as a general trend, it seems to be less of an absolute requirement.
Now it's time to talk about going down on guys. It's often said that going down on guys isn't too hard, because pretty much anything you do feels good. This is true, with a few exceptions. Things like Cosmo try to tell women that men like just a little bit of teeth in their blowjobs. This is because Cosmo hates men, and everything they stand for**. Admittedly, I can't always tell exactly what's going on once a girl has her lips wrapped around me, but I'm pretty sure teeth aren't a part of it. If there's a girl out there who wants to prove me wrong, I'm open to it. At the very least, start out lightly, and make sure you get some feedback from your partner.
One thing that really helps, especially if you're new to the practice, is to use your hands. This is, in my opinion, more crucial to do with men than it is to do with women. Jerking him off for a minute can give lips (or jaw, or neck...) a break, or you can do both at the same time - a surefire winner in my book. Keeping a hand at the base can also keep you from going down too far if your guy likes to push your head down or thrust into you. For anyone who's new to the practice, I would definitely recommend using your hands. If you feel like you're pretty accomplished and you don't use them, I'll trust you, but I have a hard time imagining a great blowjob without them.
If you use your hands, he'll look like this afterwards. Guaranteed.
I would now like to make a case for the balls. I realize they're silly looking, and it's hard to know what to do with them. Honestly, you don't have to do much. Just cup them occasionally, or move them around in your hands. Not every guy likes this, but I think most do, and it can add an extra dimension to the experience. Rubbing the perineum*** can also be quite nice. In fact, while we're getting physically closer to the topic, a lot of guys like a finger up their ass while they're getting a blowjob. This is something you'll definitely want to do with someone with whom you're pretty comfortable, and will want to ask first. This is also something you may not be comfortable doing at all, which is also just fine.
I'd like to offer a few words of encouragement for both genders. I realize I'm mentioning a whole lot of things here, but don't stress out. Remember that most everything you'll be doing down there feels good, so it's pretty tough to screw up completely. Sex should be fun for everyone involved, so relax and let yourself enjoy making someone you care about (or is hot, or happened to ask if you wanted to run into the bathroom with them) feel good. Also, hopefully the person you're doing this with likes you, and isn't going to be judging you on it. They'll be appreciative and supportive, and not overly-critical. Especially for women going down on men, there is definitely a power dynamic in play that, if you're not close with the person, can be uncomfortable. But if you ease into it, remember that you're making your partner feel great, and remember how hot you look while you do it****, you should hopefully be able to get over it. Remember that oral sex isn't supposed to be a chore or a task, or really even a favor. It should be a more like a present. A sexy present.

See what good oral sex leads to? No, not polos and golf. Focus on the happiness.
On that note, I'd like to talk to the receivers of oral sex, too. If you're getting head, you should be, well, appreciative, supportive and not overly-critical. Someone is doing a fairly selfless act for you, and it's the least you can do to thank them, and ideally reciprocate. During, though, giving feedback is even more important than for intercourse. At least the other person can do what feels good to them during intercourse. Oral is all about the receiver, so make sure they know what you like. This doesn't have to be with words. A few noises or even heavier breathing when they do something you like can do the trick. And guys, don't push your partner's head down onto you unless you've asked if you can. Girls, push your partner's head down onto you as much as you want. Well, at least if your partner is me.
*Not sure what that is? You can check out this extremely NSFW diagram here.
**Actually, Cosmo seems to hate women a lot more. This is probably the subject for another post (or probably another blog), but safe to say I don't think any woman who has a basic level of self-respect should be caught dead reading that magazine.
***While I try not to keep this blog too clinical sounding, I've never found an even remotely sexy word for the perineum. If someone finds one that isn't "taint", "chode", or the particularly egregious "grundle" that they'd like to introduce me to, I'm all ears.
****This is not a guess that you look hot while you do it, this is fact. Maybe not to other people if they were watching, but if you are going down on someone, you will look hot to that person. Luckily, all my readers are hot anyway, so they don't have anything to worry about.
Images via friedmanarchives.com, sheknows.com, mbn.com