Hi all,
There is a great deal of material out there on developing a female voice. I have read as much material and watched all the videos I could find from many sources, including purchasing one of the commercially available programs for this purpose. Different techniques may work for different people, and for me it has taken years of practice, trial and error. Within the last year my female voice has finally developed to the point where it does not give me away, which is a huge help in confidence when out. On the phone I am maam'ed 100% of the time, a tougher test as there are no other gender cues available to the listener. I should add that my male voice is very deep, and that is not a limiting factor in developing a convincing female voice.
A few tips I have picked up from a variety of sources have been most effective for me, things like using the bootom end of your head voice, starting from a yawn, lifting the soft palette at the back of your mouth, having all airways as open as possible,smile, breath support from the diaphragm for resonance, etc. One recent thing that helped it fall into place for me was a tip about imagining the sound coming out your forehead! This has made it possible for me to use the nasal chamber to create a "head voice" with female resonance..quite amazing. All the bones in your head vibrating help to make this work, so there's no "minnie mouse" character at all. The pitch will find itself naturally depending on the person, and is not a big factor in how female your voice sounds..using the nasal chamber will automatically raise the pitch since the chamber is small.
For the science behind this, check out this link related to singing: it shows where all this is, and visualizing using the nasal chamber to create the sound, coupled with all the other accumulated tips, has worked majic for me:
http://www.vocalist.org.uk/headorchest.html
Good luck!