Keep valuables (money, keys, phone) in a secure pocket. They could also be left for safe-keeping with whoever is minding the weighing room.
Avoid wearing sunglasses in the blueberry patch. They can affect how you see the colour of the berries, making them look riper than they actually are. Sunglasses also can get easily lost if they fall off into a bush.
Bring a belt to attach your picking pail to your waist. Having both hands free to hold branches and pick berries is a great time-saver.