“Getting a game design from 95% to 100% done takes as long as it does to get from 0% to 95% done.”
— Brian Henk

Board game media creators discuss how designers and publishers can work with them most effectively (video)

Lots of advice for playtesting, and its importance to the game design process (video):

https://www.youtube.com/watch?v=qBezywgh2yM