My neighbor had a boil the other day and he's from Cajun country. He does the 30 min soak method, but right after he turns off the heat, he pours a blender full of liquified onion, garlic, and lemon into the pot.
It's the best damn crawfish I've ever had, and I've had a lot.
Another thing he did was include a bag of frozen meatballs with the boil. It was a great addition if you're looking to boil something besides sausage. (It's basically cheap sausage balls).