grating a carrot." So... don't do that.
Also on the don't list: Cottonmouth blowjobs! Your mouth should be as wet and slobbery as your dog when it hears you rustling a bag of kibble.
There are rules to giving head. I'm not sure what they all are so you'll have to ask a gay man.
But there are some things women do that ruin the moment. Looking up at the dude you're blowing is lame. Either you think you're being sexy or you're trying to gauge how well you're doing. To answer the first question, if you're doing it right you'll look less like a fellating angel and more like a dick-draining demon. Second, you won't know how well you're doing until it melts in your mouth (not in your hands, please).
If a man opts out of getting blown by you don't take it personally-- you just suck, is all. A lot of women do, having spent so much time trying to stick to "good girl sex" that they've never bothered to hone their head game. I'm fully in favor of you taking a class to get it right (nothing lower than a B-, though).