First thing.. figure out what way the guy shoots, and go tie up his top arm... that's generally going to be his dominant arm, and therefor the one that you do NOT want to get touched with under any circumstance.
I'm 6'4", so my fighting style is pretty different from most guys, but I'll generally take my left arm (assuming the other guy is a righty as well), and get it under and over his left arm. Hard to describe, but I'll basically leverage his arm with my left arm, making sure it's completely tied up and my left hand is around his face.
At this point you need to go for his chin strap, if you can manage to rip it off, you've got him.. once again, because of my height and long arms I'll usually use my left arm to rip at the chin strap.. if it pops off, rip the helmet off from the back.
Now hold him at a range, keep your feet moving (so you have some balance.. worst thing you can do is stand completely still), and feed him that left hand with a few short jabs, and just wait for your opening..
And then nail him with the right
