Here are several ways I can attempt to do girls / women voices. Here are some actual examples for you...
Falsetto is the upper range that some have developed well. A soft falsetto can make a young girl voice. You can hear my young girl voice with Scarlet O' Fox here
http://www.youtube.com/watch?v=rH18lURpDV4
Take away the breathiness and a little accent and you can get a sillier girl voice like this Hippo:
http://www.youtube.com/watch?v=odDsLHS1l9Y
Here's a little lower girl voice with Verna
http://www.youtube.com/watch?v=ksG2Yl222IM
Also, you can do a good high strung witchy voice like the Wildthing
http://www.youtube.com/watch?v=fvm7dZ5Rwv4
An Older woman with a falsetto non-breathy voice, great for British or Edith Bunker type voices. Here is my Granny Gertie voice:
http://www.youtube.com/watch?v=seORsgqO2es
Lower register woman's voices / smoker-voice types are passable if they fit the character.
Here is my Edna
http://www.youtube.com/watch?v=1gDMYo00R_4
Also you can do a sexy low voice like Mae West or Terry Fator's Vickii
Have Fun! Now go voice yourself.
Ax