Eye contact - There is nothing like a a full, knowing gaze. The silent communication of submission is a turn on. And I don't think it is something that is easily faked. An old writing teacher of mine used to scream "Show me, don't TELL me!" - There is not anything particularly erotic about a sub saying that she is scared with calm eyes, but to say she is NOT scared, wide eyed and shivering...
Verbal contact - I know a lot of doms live for gags, but I don't like em myself. I like to keep a line of 'Yes Master' and 'No Master' running, and they need to be believable. It is a very important component with which the sub can sell the idea that she is a slave.
I like a very sensual session. I realize that we are not going to have sex, and I'm totally cool with that - but we're not just playing cowboys and indians either. I like to my feet, legs and chest massaged.
And for Christ sake, no giggling! There is some pretty extensive costuming and role play. Let's joke about it after the session.
A few years ago, I had a session in LA at TheChateau. When we discussed the session the sub was very businesslike and very thorough in determining my expectations and expressing her limits. But when the session started, she may as well have been a different person. Though I had enjoyed numerous bdsm experiences before this one, it was honestly the first session in which I believed that I had a true submissive. I have to admit that I was thrown off balance a bit. Her manner so suggested that I could take her and use her any way I wanted that I really had to bear down and remind myself that she was really role playing and limits had been set.
Does that help at all?