One of the most helpful things for me was seeing the movie "Better Than Cocolate" with the trans character Judy. I noticed that when she spoke more naturally she sounded fine, but when she tried to go all breathy she sounded retarded. I remembered that lesson not to overdo it.
Two things helped me change my voice. One was singing. I found it helped me stretch my vocal range and to learn how women use their voices. The second thing was realizing that it is more what and how i speak that matters, more than the pitch. To me, there was a whole other language to learn called Girlspeak. Listen carefully when you're with your girlfriends...women use different words with different inflections than men do.
I hope some of what I wrote here will be helpful to you.
