Beyond the physical, I find that the way you carry yourself -- I guess, your aura -- makes a difference. It's hard to explain, but confidence in yourself as a woman makes a big difference. You don't have to be a super model or look perfect, because there's a lot of variation; however, if you're going around constantly wondering if people know about your past, or think about any "maleness" it will draw attention and have people have a second look at you. I've seen it countless of times with many other TS girls.
You can have the perfect looks and the perfect voice and the perfect mannerisms - but if you don't know if you're woman, others will sense it. There's a difference in knowing and wanting to be or trying to be.
I know I'm a woman, and that's not questioned. I might not be perfect, and sometimes I will nit-pick myself. I will think that my voice sux, or that my looks aren't good enough. I don't know if I'll ever get over that. But, at the end of the day, I know I'm a woman and I've never been questioned on that. I own that.
I explained to one friend that it's like if you decided to go for a fashion that you don't normally wear -- say, you normally dress pretty regular, and then decide one day to goth it up. Then, you go walk around the mall all gothed up dressed different than yourself -- people are going to notice you and get a sense that something isn't right. You'll probably run into other goths who'll probably spot you as a poser right away.
So, really, a lot of it comes down to your own psychology and knowing who you are and owning it. Others will sense that and you'll be fine.
Not that the physical things don't help, such as looks, voice and mannerisms. Those are great things to work on because those are tools for you to use to become yourself. If you don't know yourself, however, then those tools will have no foundation to build on. Also, look good for you and sound good for you - it's about you. Don't do it to try to impress others or try to make them accept you, 'cause people are going to sense that and wonder what's up. Hold yourself up high, be yourself, and everything else will follow.