Another view from across the fence -- getting read happens, even to post-op MTF TSs.
Obviously physical features
can be a giveaway, but I'm willing to bet it may be body language and mannerisms. If you haven't done so, read Norah Vincent's
"Self-Made Man," which chronicles the writer's 18 months of posing as a man.
Vincent has a body you guys would probably kill for (5'10", size 11 feet, reasonably deep voice), so she did have some advantages, but she found that over time her mannerisms became so masculine that she ended up skipping most of the things she'd done to make herself look like a man and people didn't read her.
That said, people invariably thought she was a gay man, because she came across as effeminate (which shocked her, since she considered herself pretty butch). A lot of it was that she didn't know "The Rules" for guy-hood -- all the unconscious stuff we learn growing up. So she did things like referring to another guy as "cute" -- which literally stopped conversation.
It
is hard because it involves all sorts of things you usually never think about. It's the same kind of issue we MTFs face in learning "femininity as a second language" (as Fecility Huffman put it). As far as body language and vocal patterns, you might take a look at MTF tutorials on it, and do the opposite. The short tutorial on speaking is that men typically speak in a monotone and use volume for emphasis -- which it the opposite of how women typically talk. Men are also far less spontaneous -- that was a major lesson Vincent learned. As far as the style of talking, take a look at linguist Deborah Tannen's
"You Just Don't Understand: Men and Women in Conversation" for insights on the different "meta-language" men and women typically use.
If you are convinced it's your face, it a couple of things.
First off -- facial hair. Men don't have the downy kind of facial that women do, so get out those razors guys.
Second, there are differences in the facial structure of males and females (hence FFS). You might not like this suggestion, but some make-up might help to do a little contouring. MAC's Harmony
blush is a good neutral that's subtle -- I use it for contouring myself. I'm not sure if there's any tutorials on FTM facial contouring, but you could try looking at some of the MTF tutorials and do the opposite.