I usually don't comment in this thread, but Mandy, it's probably proportions. Things like head height, width, shoulder width, bulkiness of muscle, those things get you clocked. I clocked a transsexual at Walmart that is otherwise completely beautiful just based solely on chin width and forearm definition. Turns out she was a friend of a friend who my friends always clamor about how beautiful she is. I agree, the girl is gorgeous and I'm sure she doesn't get clocked often at all, but things like this get you read and unfortunately they're things we have no control over.
If it's not that it's posture. Women have a pelvic tilt that men do not. HRT may or may not change this naturally (alignment of the ligaments can change to varying degrees with HRT over time... happened to me). Basically, the penis (or where it was) needs to tilt more towards the ground than normal. At the same time, women lead with their hips, not with their shoulders, and they also walk with as little shoulder movement as possible, letting their legs and hips do all of the work. Women also keep their shoulders relaxed but cocked back slightly.
To get the cocked back shoulders, try this: imagine there is a very thin bar going right through middle of your sides where your elbows naturally fall. When walking or standing with your arms at your sides, try to keep your elbows behind that imaginary bar. What this does is forces your body to pull your shoulders back. This coupled with the pelvic movements will bring a more feminine posture.
