I think that most guys are easy to please when it comes to sucking their cock. Sure, you might find yourself the occasional connoisseur, but generally if you go for quantity over quality you can't go wrong. But, if you want to blow his mind when you blow his cock, here's some tips. I've got most of my information from asking other people, both guys and girls, and I've included stuff I've learned in my experience as a cock-sucking slut!
First of all, the general opinion seems to be that unless there's a pressing reason (like you're sucking off total strangers on a crowded train) it's preferable if you're naked. Particularly if you've got nice tits (and let's face it, boys are less fussy about the tits they can get in their bedroom than they are about the ones in porn), topless is almost a minimum requirement. There are a couple of advantages to being naked when you're giving a blowjob. Firstly, it's easier to clean up -- just rub anything that lands on you into your skin. Sure, you'll never get a nice Monica Lewinski-style souvenir, but at the same time you'll save money on dry cleaning. Secondly, it means easy access so you or he can play with your bits and pieces -- you'll both love it, more on this later.
Now, obviously getting naked before giving head will decrease the spontaneity a bit. Spontaneity is something a lot of guys like, and I think there are a few reasons. Firstly, the idea that you can't wait to drink the cum straight out of him will probably drive your man wild. I remember watching in awe as my best friend dropped to her knees at a music festival once, her man loved it so much she only just got her mouth around his dick in time to swallow his load. I think a spontaneous blowjob also has a subtle psychological effect. If you randomly whip out his cock and suck him off, it will seem more likely to him that you'll do it again some time.
So to put a bit of that spontaneity back in, you could try a couple of things that work for me. Obviously, you could take advantage of already being naked, if you're changing clothes or having a shower. Another thing I've found drives most men wild is to drop to your knees, start sucking on their cock, and then tear your clothes off. Finally, you could do a little bit of planning and be wearing something that will come off quickly and easily as you move toward his big, throbbing member. You could, of course, suck him off fully clothed every once in a while, some guys love it like that, some like to have a view.
A very important facet of your blowjob is what I will, for want of a better word, call presentation. I've found that there's not much turns a man on more than the idea that you can't resist sucking on his dick. Paradoxically, a bit of teasing is also a big help, particularly if you're trying to make and impression. The old sucking on a prop trick is clichΓ©d because it works -- don't be scared to use it. If there's nothing suitable readily available, suck on your finger. This is extremely effective if you do it in public. Just keep your eyes locked on his, so he knows the prop is standing in for his dick, not someone else's.
In a similar vein, give him a big sloppy kiss and pretend his tongue is his dick. Suck it into your mouth and run your tongue over it. Some guys like this more than others.
Once you get him alone with plenty of time, give him a show. You or he can take your clothes off, but if you want him to do it you might need to get it started to give him the idea -- men can be a bit slow sometimes! If you're a confident girl, give him a bit of a striptease, with or without music, finish up with a few sexy poses, then get out his dick and chow down. If you're a bit less outgoing, or you're shy, take your own clothes off, but do it slowly. One thing that used to help me was imagining your hands belong to someone else. Move a slowly as you can, turning occasionally. Caress your own skin like a lover should. A big teasing tip: remove your underwear with your back to him, but turn between bra and panties so he gets a look at your tits. Pinch your nipples a bit for him, I've never met a man that didn't work on. If you're insecure about your body, do all this with the lights low; guys have a pretty good imagination, so chances are he'll remember you looking sexier than you think.
So now we're at the stage where you're naked and he's starting to go crazy with lust. You need to get over there and start sucking, and you can just do that directly if you want. This works well in terms of making him feel irresistible. I would recommend saying something like, "Get that big dick out, honey, I want to suck you dry," as you advance on him. Try to time it so your knees hit the floor as his cock pops out of his pants, that way it's more like a porn movie, and he'll love it.
To keep things in your control, you need to keep him mesmerised, so I'd recommend slow and sexy movements. If he's standing I like to walk over to him, swaying my hips, and grab his belt. Pull him up against you, press your tits into his chest, and kiss him as you undo his pants. Get down on your knees as you push his undies down; kiss down his body if you want, I would recommend looking into his eyes.
If he's sitting I like to crawl over to him. If he's on a chair I crawl all the way, but if he's on a bed I'll walk to the bed then crawl up to him. As you crawl, arch your back downward like a cat stretching. Try brushing your nipples on the floor. This leaves your ass sticking up in the air, and the crawling motion will make it sway enticingly. When you reach him, stop at his dick, give him a lustful look and start getting into his pants.