I imagine actresses, porn stars, and on occasion girls I know.
For instance, in "Sappho's Amulet" I based the following characters thusly:
Emily, the waitress: Jenna Jameson, pornstar
Pandora, the previous amulet-keeper: Veronika Zemanova, pornstar
Ms. Hayes, the executive: Famke Janssen, actress
Darlene & Danielle, the girls upstairs: Lisa Kudrow, actress (both of them - they look alike)
Fawne, the stewardess: Fawne, a girl I once knew, though unfortunately not carnally. She, in turn, looks a little like Winona Ryder, except with short hair and green eyes. I can't tell you her hair color, because it changed every week and I never knew what it actually was.
For "Doing the Sorority" all characters created by me are based on girls I knew in high school and college.
Unfortunately, CHYOO talk doesn't allow links to outside sites, but if you look up any of the unfamilliar names above on Google or something, you'll find them.
Unless I'm basing the characters on someone I know offline, I surf the internet for photos that inspire me.....I actually find amateur photos more stirring that professional entertainers or porn stars....Must be something about real people doing very naughty things maybe.....*smiling*