Whether you are using a prep service or not, a prep technique that hasn't been discussed much here is a review of your own history, both flying and non-flying. After you've gathered all the examples of questions and scenarios, look at your own resume and logbooks.

If your flying career is in its infancy (i.e. second career flyers or less than XXXX-hrs. FAR 121 (just a number), look at your resume for non-flying jobs where you were in leadership positions or had to deal with customers. I did non-flying military and found some good stories of dealing with "co-workers" and some good leadership examples. I did retail management before flying and that had a good amount of co-worker, problem-solving and boss/subordinate work scenarios. If you don't have a good 'flying' story for a particular question, but you've got applicable real-world experience, use it! It'll show the interview team your flexibility and that you reviewed your entire history during your interview prep. They want to know you as a 'whole.' They'd rather know how you acted from personal experience rather than a theoretical 'what if'.

For your logbook review, look for memory-joggers. You'll look through a dozen pages of nothing-burger-boring flights and then stumble across a "I forgot about that one"-flight that you can add to your 'tell me a time when' question database. Many pilots have notes in their logbooks to remind themselves about 'events'. Go find them. Even if you don't find specifics, maybe you'll find a tail-number that jogs a memory: "man, what a piece of junk that plane was. It almost killed me by..." or "that was the plane I almost flew through a thunderstorm in". The whole point of the personal logbook review is not the hours contained within, but the memories. Fill up a notepad of your memories. They're the best information to build your stories.

Also, I can not agree stronger with the posters stating to get your app reviewed by as many eyes as are willing to review; either paid service or not. After you've scrubbed through your own application a hundred times, there's no way you'll see errors. Little things like punctuation and consistency throughout the app show 'attention to detail' that you'll miss after getting bleary-eyed looking it over.