Feeder here. (I came across this post when I was bored but not sleepy. I meant to post a casual reply, but ended up writing like a whole blog entry on the topic. Maybe someone will be bored enough to read it...)
Two thoughts come to mind about "turning" a dude into a total cocksucker—or, maybe I should say, helping him realize his potential he already has for being a total cocksucker.
1. Giving firm, controlling instructions and guidance about the process/activity of sucking dick. I think most guys (even future cocksuckers) don't realize that sucking dick is, in and of itself, a sex act. They've always learned to see it as just a method, a means, a technique for busting a nut. In much the same way, all boys start out thinking of jerking off as just a means to blowing a load, not something you do for its own sake. Often when there's a dude on his knees in front of me, and I give him the cock he craves to suck, he starts furiously bobbing up and down on it, and trying to jack me off as well, because he thinks it's his duty to get the load out of my balls as soon as possible. As if that's the whole point of sucking dick. And if he's a true cocksucker, he does enjoy that—but he's not really experiencing the act of cocksucking. I usually have to introduce him to that experience by first holding him back, restraining him from going for my load, and instructing him to give my dick the treatment it deserves. I put the cum shot off the table, until he falls into line as a serious cocksucker. It's a lot like (no offense) training a puppy to heel, or to display controlled behavior before getting a treat. Soon enough, a real cocksucker realizes that this is all they really wanna do, ever! They could never grasp it before now because they were sort of skipping over the act of cocksucking, leaping straight to the idea of draining the guy's load.
2. We simply do not discuss his sexual arousal at all. I'm not into degrading his sexual feelings: I'd never act like his own cock was disgusting or pathetic (assuming he has a cock—although if I've ever even seen it, it was in passing or accidentally). Nor do I dig aggressive denial/depravation play (cock cages, chastity gear, etc.). The point is that his own sexual sensations are completely irrelevant to the act of sucking my cock. This has to be ingrained by me as a feeder, because most guys start out thinking there's something wrong with them if they enjoy a sex act where they're totally subordinate to another man's pleasure. But that is what they enjoy; that is what cocksucking is like. It's just not a mutual or reciprocal activity. I'll see inexperienced cocksuckers "pretending" to jerk themselves off while they suck dick, although their little penis is limp as a noodle. And they're already fully enjoying themselves anyway, just by getting to suck a cock. The problem is that their own dick is totally irrelevant to the situation—they just think they're "supposed" to be seeking some self-gratification. Same goes for when a guy obviously just wants my dick in his mouth, but he sheepishly mentions the possibility of me fucking his ass. He thinks it's weird if he just wants to suck cock. (Cocksuckers often tell me they like to jerk off while thinking about sucking my cock, at some totally separate future time. That's perfectly appropriate.) Anyway, by firmly refusing to consider his own sexual arousal, I find a cocksucker achieves clarity and focus, and realizes that this activity—servicing a man's dick—is the one thing they are devoted to, the one thing he truly desires to be doing.