Mike Tuchscherer commented on one of the Reactive Training Systems blogs and made an interesting suggestion:
“Try to pack your lats down more at the start. The key to keeping your back straight isn’t trying to keep your back straight — it’s packing your lats down. It’s a flex of the lats that squeezes the bar in tight against your shins.“
As someone who has had trouble keeping their back flat on the deadlift I can say this is an intriguing notion and something I’ll have to play with!