Just some ideas…
- Articles, videos, and podcasts that portray ClassicPress in a positive or neutral way. (from here)
- In your advertising material, where stating that your plugin/theme is compatible with CP
- In your advertising material, where stating that your web agency can use CP to build sites
- In your advertising material, where stating that your hosting service supports CP
- In a website (see logo usage guidelines to see if the site is allowed to), stating that is built with CP
In any case:
- You must follow logo usage guidelines
- CP logo must not be prominent
- The logo must be used in a way that doesn’t suggest ClassicPress endorsement or affiliation
Possibly in a better english than mine 