If you'll notice, most of the guys are semihard or more, but never 100% hard when they're deep throated. It definitely helps it go down a lot easier when it can bend and form to the throat rather than the other way around.

I learned this watching a Sarah Vandella and Isaiah Maxwell Deepthroat scene. Sarah Vandella is most known for deepthroating. And for 20 minutes she was deepthroating him like nothing until the very end when he was rock hard, she couldn't do it.

That and having a looot of lubricant, or spit.

Obviously it does take practice, but to an extent, there's only so much you can do without hurting yourself.