Keep a growing collection of great contributions posted in your community.
If you see a good question, a great answer, insightful blog post or other great contribution tag it in Evernote (or just copy and paste the line into a doc).
Now drop these examples into your community journey.
In the newcomer guidelines or welcome message, show examples of great questions and what makes them great.
In your documentation for MVPs, highlight great responses posted in the community in the past and what made them great.
Examples like these really help members infer how long questions and answers should be, the level of detail they need to provide, whether to use bullet points, screenshots and video. They can also infer the right tone of voice to adopt.
p.s. This video might also help your member journeys.