The best way to not scare off someone who is trepidatious is to communicate with them and go slow. Talk about fears, talk about expectations, talk about past experiences and future ones.
When begining a scene they might be worried about. Take the approach my dentist uses. Explain exactly what you are going to do next. i.e. "Ok, here comes another drop of wax", "Ok, here comes another strike with the flogger, this time harder and faster".
You can train a sub to expect what you are going to do by providing some repetition to it as well. Using rhythmic music during a flogging scene and striking to to beat would allow them to know when to expect the next impact.
And I'm of the same school of thought Oz is. I don't try out my toys on myself.