How to Guide Blow Job
The number one blowjob mistake: letting your teeth hit his cock. I can't stress this enough. Ask any guy and he'll tell you again and again: nothing is worse than a blowjob that is "all teeth". In fact, most guys won't admit it, but their idea of a perfect blowjob would be getting sucked off by a gorgeous woman with removable dentures. Teeth can nick and sc**** the sensitive skin of the penis. Open your mouth wide enough that they stay away from his cock, or suck your lips in over them to cushion the rough edges. If he's wearing a condom during the blowjob this may provide a slight buffer, but don't count on it.
Going too fast
Some guys enjoy a quickie, but unless they're in a hurry to get back to work or back to their wife/girlfriend, most men like to savor the experience of getting their cock sucked. So don't rush them. It's not a race to see how fast you can get him to spurt. Take your time and make it last. Concentrate on different parts of his penis, or his balls, then go back to basic sucking and stroking. At the end of a luxuriant BJ session, he'll blow a much larger, more satisfying load.
Sucking too hard
Hoovering him like a dustbuster may seem like a good idea in theory, but there's a wrong way to do it. Don't fasten your mouth around the head of his cock and suck like you're trying to get a triple-thick milkshake though a straw. Guys' dickheads are the most sensitive part of the penis, and uncut guys are even more sensitive there. The proper technique to hoover is to take more of his cock into your mouth and apply moderate suction.
Crushing his nuts
Don't grab his balls and squeeze like you're trying to crack walnuts. This is a move you want to pull on a would-be r****t, not a guy you're trying to get off. It's painful. Many guys like to have their sack rubbed, licked, or gently tugged during a bj. You can even take a break from sucking his cock to take one or both of his balls into your mouth. But don't yank on the f****y jewels, and don't put them in a vise grip.
Barfing and gagging
Throwing up while giving a bj is definitely a turn-off, although sometimes this happens to the most well-intentioned cocksuckers. What happens is they get overenthusiastic, take the cock too deep or hard into their throat, the gag reflex gets triggered, and then it's all over. Or once the cum starts hitting the back of her throat, things start flowing the wrong way. Don't try to deep throat on your first BJ. Everyone will be sorry. Practice first on dildos to get your gag-reflex under control. Or just work up to it at your own speed.
Putting his cock in your mouth and then doing nothing is as bad as overdoing it. Most guys don't want to feel like they're face-fucking a cadaver. You need to work it at least a little. A total lack of effort or enthusiasm on your part is a real buzz kill. If you don't know what to do with your tongue, at least bob your head. After a while, he may want to take a more active role and have you relax while he fucks your mouth, but you need to give him a little encouragement to get him aroused.
Not using enough pressure
Wimpy blowjobs are no fun. Be firm and suck him like you mean it. Lapping his dick lightly will just make him feel teased. Licking his dick like it's a lollipop or popsicle may be fun to get warmed up, but then you have to get down to business. Use a firm (but not too firm) hand to work the shaft of his cock, while you get your mouth all over his knob and caress it with your tongue.
Most guys want you to swallow and think that a BJ really isn't complete until his load goes down your throat. The only exception is if he gets off more on spraying it on your face, tits, or somewhere else. If you really don't want to swallow, this can be an acceptable compromise. Symbolically, swallowing shows that you accept him and his manly essence fully. Although we can't endorse unprotected bodily fluid exchange in any form, if you happen to be giving an unprotected blowjob (i.e., without a condom) and you end up with his spunk in your mouth, do not retch his cum into a wastebasket in front of him. If you really feel you must spit, do it discreetly into a towel or tissue.
A good BJ involves a lot of saliva. Don't be afraid to get a little sloppy and drool on his cock. If your mouth is dry from nervousness or whatever, there will be too much friction and stickiness to give good head. If your mouth is dry, drink some water and rinse. Try to work up a good mouthful of spit before you go down. Suck on a hard candy to get your juice flowing, or suck on an Altoid to give him a mentholated BJ.
Don't put his cock in a deathgrip. It's fine to wrap a hand around the shaft, but don't clench and for god's sake, watch the nails. Just hold it enough to keep it from flopping around and shuck his pole with your hand while you work him with your mouth. His dick shouldn't have a handprint on in when you're done.