If I don't feel like wandering over to the computer to look at my subscriptions to ClubJenna or CumOnHerFace.com, OR if I don't feel like watching OnDemand porn ($14.99 for no money shots?  Ridiculous.) then I think about the following in no particular order:

a) Previous sexual experiences.  I can nearly always masturbate myself into a frenzy thinking about my first one-night stand my junior year, having sex in my parents foyer with a summer fling, or being fucked on the hood of a boyfriend's car on the side of a deserted Lawrenceville road.
Most recently my though thoughts usually turn to my favorite clients.  Memories of my very first experience in the business usually excite me the most because I remember how excited I was at the prospect of the sordid things that I was doing with essential strangers.

b)Other women, particularly high volume pros.  
When I was in an agency situation I was in an enviroment designed for nothing but me experiencing orgasms all day from a variety of men.  I think about pretty pros getting screwed all day by a barrage of sexually frustrated men.

Which leads me to:

c) The gangbang: I've never experienced this but often my  masturbatory experience is enhanced by thoughts of group sex with multiple men (hence my subscription to COHF.com.)

d) The professor of my dreams:  I'm a sucker for nerds; I have a wicked crush on my favorite professor.  Cliche-but I often think of, you know, screwing him in a study room or a stairwell in the library.

I masturbate often and usually it's not to pornography, so I have lots of thoughts.

A better question?  Ladies-*HOW* do you masturbate or how often? {LOL}