I like doing it if I'm really into the guy. Which requires him to be either 1) really hot, or 2) really smart. If he's both, I'll gag myself on his cock all night long.
Plus it helps if I am 1) drunk, and 2) haven't eaten anything solid that could come up.
Also, he needs to be respectful. I if I squeeze his arms he needs to back the fuck off of putting pressure on the back of my head. I can just picture the expression on my face if I die from having a cock in my mouth.