Tell HN: Don't worry about people stealing your ideas
"Don't worry about people stealing your ideas. If your ideas are any good, you'll have to ram them down people's throats." --Howard Aitken
The other advise about hiding ideas, which has nothing to do with this quote, is that its the execution of the idea which matters, not the idea itself, which is also very good advise. But again, someone else who steals your idea may be in a position to execute it better than you (more funding, more resources)
Your best bet is to make your business an attractive target for acquisition. That way you cash out early on the basis of huge potential future revenue.
Rinse and repeat ...