Disclaimer: I have never operated a farm stand.
However, I do a lot of reading of marketing materials and I would suggest a couple ideas.
Place a mirror where everyone will see themselves. Research I have read shows this will improve your results.
Place signs that thank your customers for their honesty and ask them to refer their friends. People like to be thanked and like to help.
Give your customers permission to write an IOU. Trust goes a long way.
I'd like to read from experienced farm stand marketers, what proven ideas, practices and techniques can you share?