While my explanation may be a bit technical, understanding the anatomy of the act helped me do it.
There are many different shapes in which a penis may be found. Carefully observe the penis under well-lit conditions to determine its shape, especially its curvature.
The curvature of the penis has a lot to do with how the deep-throating newcomer should approach their goal of getting as much of it in their mouth as possible. Follow the curve!
The above picture shows a standard person's airway, it has a curve from the back of the mouth into the throat and straight down from there. By keeping that curve in mind, you can work with your body, not against it, to achieve deep-throat status.
For example, if the man curves upward toward his belly button, then climb on top with your bottom toward his face and tackle his tackle from that direction. This lets the natural curve of the penis work with the natural curve of your throat. Not to mention they're getting a great view and who doesn't love a two-fer?
Before starting this challenging sex act, let your partner know that you're trying something new and it may take a bit of practice. Personally, I do not like having him cum in my mouth, so I want to know when he's getting close. The threat of snowballing is usually enough to make this stick.
Start with lots of licking and, to be honest, slobbering over the target penis. No, not the one half way across the room, the one you're trying to deep throat. This helps lubricate it as you prepare to slide it down your throat. Suck a bit, lick a lot, have fun here (that is the entire point of the exercise.)
Slowly, gently, bring it deeper into your mouth, following the curve of your throat and the penis as much as possible. Take things easy, if you start to feel gaggy, back off. Do some more licking as you wait for that reflex to calm down. Trust me, he won't mind. It takes practice over multiple sessions for many to be able to get their throat to accept the intrusion.
So, you've managed to get the penis in your throat, great. Remember, you will need to breath, so back off the penis while needed to get your breath back. Passing out from hypoxia (lack of oxygen) isn't very sexy to most people and is a great way to ruin the moment. Once you're ready again, repeat penetration. Swallow while it's down there, it produces a wonderful ripple-like sensation (or so I'm told).
If you're up for the whole thing from suck to swallow, have fun. If not, I imagine he'll still appreciate a good deepthroating as foreplay.
Best of good luck.