Be Original
Nobody wants to hear the same plot over and over. Having a deliveryman or a repairman come by and seduce the housewife has been done before. Think of something more original. It could be something that's happened in your everyday life, just spiced up a bit. Use your imagination and be creative.
Or you can think of a sexual fantasy that you'd want fulfilled. This may give you some good ideas or at least a good starting point for your writing.
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Give the Readers What They Want
The reason why "50 Shades of Grey" has garnered so much buzz is that it's wish fulfillment on behalf of the readers. The book is giving them something that they're not used to getting at home. So the trick is to come up with a sexual fantasy so off the wall, something that the average couple would be unlikely to engage in.
Erotica is about obtaining the unachievable. Erotica allows readers to live out their fantasies, which makes it so interesting.