I'd suggest looking for an old copy of "Melanie Speaks." I have that one, it's good on the lesser-considered means of communication.
E.G., the Pace and Intonation of How a woman Speaks?
(Try stressing the capitalized words, and raise the tone at the end of the sentence, like asking a question? See how it feels different?)

I'm not sure it's still "in print," but it might be available here and there. If not, I'll see what I can do to find it again... Haven't seen it in years!

It's more important to SOUND FEMALE than to sound girlish. Husky-voiced women are around, it's the breathing, the location you speak from, the vocabulary you use, and the phrasing and intonations, which will make you pass or fail. ;-)
A breathy, lilting, enunciated pattern of speech will help. Tell a story, don't just convey information. (See also my unpleasant post elsewhere on "the words don't matter." You're not talking to convey information, but to connect, to associate, and get opinions and cover or shade messages.)

I've heard good things about "Deep Stealth Productions," but never bought anything from them.

If you have a large, booming voice as a man, you'll want to constrict it a bit in your throat, too. :-) STOP talking from the chest. Control your breath, and ASK, don't demand, don't give orders. Women are held to a very different standard there, so tell people what you want, and let them figure out how to do it.

I'll go looking for the tape (VHS) tonight. Might get lucky and find the CD, too....

-Dianna