There are three steps to developing a female voice:
1) Learn the techniques 2) Practice 3) More practice!
These lessons are a combination of my own techniques, a session with a professional voice coach and singing lessons. I have eight years of radio broadcast experience, sang in a rock band and in an all-women choir (the latter two stealth). If you have singing, radio broadcasting, or character voice experience, these techniques should be easy for you.
I highly recommend finding a voice coach. It there are no voice coaches in your area, find someone who teaches singing and tell him or her you just want to work on the upper register.
Recording and playing back your voice are important! There are many nuances that make up the female voice and you can only hear them if you playback your voice (When a person hears his, or her, voice played back for the first time they're usually surprised at how different it sounds, because it's resonating in your head and sounds different from what others hear).
Also recommended is recording your phone calls, either with your computer or smart phone. They will be invaluable to your learning process! The phone is an important test since the person you're speaking to has no visual clues about your gender (Check your local laws. In some states it's illegal to record a phone conversation without the other person's knowledge).
Actors are taught warm-ups to get them ready to read scripts and they are applicable to learning a female voice, such as saying the vowels (a-e-i-o-u) and over-emphasizing them by making exaggerated mouth movements. This will help relax your mouth and jaw muscles and achieve the clear enunciation and modulation of the female voice.
The main difference between a male and female voice is pitch. Don't believe anyone who says pitch isn't important! Males have deeper voices due to longer and thicker vocal cords. It's like the difference between a flute and an oboe. You want to move the resonance out of your chest and lower throat and up into your nasal passages and head.