I'd like to add a few points for your readers.

1) If you are worried about hitting your gag reflex a good way to avoid that is to take the man as far as you are comfortable and then wrap however many fingers as necessary around the base of the shaft so that your lips are touching them. You'll have set your limit that you won't go past as a result and can relax knowing that you won't hit your gag reflex as your fingers will prevent taking it deeper.

2) Go slow. Forget every porn video you ever watched. Going at it like a jackhammer is not erotic. Give him time to build.

3) It's better from the front than from the side as the full tongue goes down the underside of the shaft which gives a better sensation. Also, he will cum a bit more quickly if you are worrying about fatigue.

4) After he ejaculates keep him in your mouth and continue to suck gently and I mean gently. Don't try to tease the glans at this point unless you want to make him jump. Use your fingers around the base of the penis to milk that last bit of semen out of it.

5) Don't pull you head away when he starts to ejaculate. If you aren't a swallower switch to just hands at an earlier moment and not at the last second.

6) If you can. reach up and stroke his nipples as he's getting close.

Hope this helps!