Hiya, yeah that was the problem I had for ages!
Basically the part under your chin drops down and then he can get past your mouth in to your throat. You throat is actually very stretchy and when that part drops its a big enough gap for even a thick guy (my guy was 7" around the shaft at the thickest point)
I assume you know you need to breathe out.. you'll never get him down otherwise because the lungs force air back up and you just cant get past it.
Right now.. that part under your chin loosens real easily the more you do it. For the first time its not so much tilting for a straight neck as it is about pushing your head forward. For example an easy way to explain.. if you were kneeling in front of him - take a whole step back and by leaning in to him this way you are pretty much at the perfect angle.
You can see me doing it from another really nice and easy position in the link just scroll down a bit. You can see my head goes forwards as oppose to fully tilting (I hate to use this example but like doing the funky chicken with your head is what I mean but pushing forward -
http://mineinprivate.blogspot.co.uk/2012/01/how-to-deep-throat-from-someone-who-can.html?zx=387e4e46da94db83Also, try starting a yawn but don't do a full yawn. Put you hand at the top of your throat and you will FEEL that part Im talking about poke out. You can poke it in and out at will by half yawning.. When you get him to the back of you mouth just before you attempt to go down if you widen your mouth (some more) this does it too.. Its a bit of a knack but you will get it now you know what needs to happen.
Once you've gone down once, its like ahhh eureka! and it goes down easily. But as well, if you dont do it in a while it all tightens up again. (I guess that is good news really)