I find it a little elitist that you're only asking the wonderful authors this question. We bad authors are people too.

I'll largely second what was said by Rollin (and, by proxy, Stephen King). Inspiration is all around you. For a spanking story, you can look at any mundane situation around you and think of how to kink it up. For instance, some years ago a date told me she was on the college paddling team. It took me about 0.02 seconds for me to figure out she meant rowing, but in that fraction of a second I got the idea of paddling as a school sport, and it became my most popular story on the LSF thus far.
For characters, look for interesting personality traits on TV or in people you know and work them together to make new characters. That goes for physical appearances too; sometimes, I just need to see an attractive woman for my mind to start building a story around her doppleganger. For instance, one of the main characters from my aforementioned "Paddling Team" is a physical and personality hybrid of several peppy, athletic girls I went to high school with.
As with any genre of writing, an important part of inspiration is to read other people's stories. I'm not saying to steal outright, but inspiration from an existing story can lead to something very different. For instance, I read a story on another site in which a sorority housemother spanks one of the girls to help her get the attention of a boy. I took that germ (girl spanks other girl to help her attract a specific male audience), but changed the personalities and threw in a couple of twists that turned the story on its head. The result was my "Backfire" story, which ended up having several sequels that took it away completely from the source material.
In short; look around for details, mix and match them, and throw in your personal kinky tastes.