Sell support contracts. Kind of similar to MongoDB's strategy, but they were too late to the party. Also the "support" open source business model doesn't always scale well and works better when the product quality and documentation isn't so first rate (like Rethinkdb's is).
I've always found that to be a perverse incentive model. I don't know how to do any better though. Maybe shaming companies that use OS projects heavily into donating? Not really sure how to scale that either.
