A lot of good advice in this thread. I'll actually echo the advice about making him feel like it's about the PROCESS and not necessarily the END RESULT that you want him to enjoy. "Don't rush yourself, just enjoy it" is the best thing a guy can ever hear during oral. If a guy feels rushed (which can either be from you not seeming to like what you're doing or if you keep hinting at him having an orgasm), he will feel pressure that will not only make it harder to climax, but it will take him out of the good feeling of the moment and into his "focus zone." True, you want to end it with him having an amazing orgasm, but it's best to let him know that you want to keep him at that state right before an orgasm as long as possible. Guys love to get off, but it also means that it's over, so sometimes it can be a little anti-climactic. If a guy know that he can close his eyes and lay back, or prop himself forward and watch, and really sink into the moment of things, it's going to be much better than oral sex that just pulls out all of the tricks to make him finish as fast as possible.
So endurance is key, and the best way to handle this is to alternate your hand and mouth. And it's okay if you need to stop and rest for thirty seconds or so (he might actually like this, since it can delay the orgasm), as long as he's not duped into believing that you're going to quit and leave him hanging.
Something that some people like (has worked when I did it to women, and it can work on guys too) is to reach up and hold his hand during the middle of it. It establishes a personal connection that will give him a rush of love for you and the intimacy you share. You don't have to keep holding hands, but just a small acknowledgement that it's the two of you together in the act.
And the last thing, my biggest pet peeve about oral...Actually suck on him. Sucking feels amazing; it's what differentiates oral from other types of intimate contact.