I am Pre-T and I am stealth.
My work supports my trans status and thus I am able to be stealth at work. Only managers have possible access to my female identity and as far as I know only one lady knows and that's because she signed some applications I had and I trusted her most to give them to. She was very kind and actually more concerned herself about me exposing my female staus

It was very sweet.
At school I am stealth to everyone besides teachers. I let them know that I prefer male pronouns and my male name, Sebastien. So that helps with no one knowing that I'm a female. As for people who knew me in the past that's taken care of somewhat since most understand that I am a transsexual etc.
Exceptions would be that neither my parents or my SO's parents know that I am a transsexual. They just assume me a very butch lesbian

And my sisters know, but they can't actively do things to support it since they have to walk around my parents, if you catch my drift.
But as I am sure you can tell from my avatar, I am not the most feminine looking person in the world. I have passed for male since before I even knew what a transsexual really was. Though I always enjoyed it. I find it funny to look back and realize what I didn't realize then. Anyway, I agree with Shaun that it isn't such a terrible task so long as you don't look terribly feminine and don't have a terribly feminine voice. Neutral is the gateway, but anything more feminine than that isn't too good.
So I guess just try to be pretty androgynous, train your voice to go a bit lower, and make sure you've got masculine mannerisms. That should help when T isn't an available option. And don't let not passing put you down. Just keep trying.