“Don’t stop playing published games. Make it a point to keep informed of what’s going on, what new tools other people have invented. Only playing prototypes will leave you in an information-rut.”
— Tim Rodriguez
“Just do not rush something out of the gate because you are anxious for it to be completed. I see so many people that are so excited about their idea, and it really is a good idea, but it suffers from a lack of development because it is put out too quickly.”
— Dan Letzring
“You cannot start a play-testing session without knowing what you are testing. You must exactly know what you are checking that day. What question you are looking to answer.”
— Ignacy Trzewiczek

Pitching to a publisher, start to finish (video)

“The optimal move/choice/strategy/character should also be the interesting one. NEVER make players choose between “fun” and “effective”.”
— Keith Burgun
“Don’t spend time explaining what’s changed from previous versions. Just teach the game in its current form.”
— John Brieger
“Ninety percent of the time, my advice on particular game design and development issues boils down to this: Do the work. Avoid shortcuts.”
— Eric Lang

Six tips for writing better board game rules (video)