Too many guys forget when deepthroating that your still giving a blowjob, and your mouth & throat is not just a deep warm receptacle. There's a huge difference between getting it all the way down and giving proper throat job. Once a guy is balls deep the real work (fun) begins.
With him buried to the hilt the tip of your tongue is by his balls and can't do much to his cock, but definitely should be paying attention to those nuggets! You've also got all those muscles in your neck to work the head and shaft by rocking your head, pushing your tongue to massage under the shaft, and deep short one or two inch higher-speed thrusting. As you back off and have more control, tongue swirling every time you bring yourself up towards the head, and as your going back down, is important.
Think about the best non-throat blowjob you've ever gotten, then do exactly the same thing PLUS the deep work. And keep doing it until he cums or pulls you off. Yeah, it takes breath control and timing, as well as the ability to fight past the gag reflex, but with practice those will come more naturally.
Did I mention that I love to give deep head to very well hung guys? ;-)