Do not use blueberries packed in heavy syrup; fresh or frozen (unthawed) berries yield the best texture.
Use real butter instead of margarine for the best flavor and richness.
For a cinnamon-apple variation: Substitute blueberries with 2 1/2 cups of chopped apples. Mix 1 tablespoon of cinnamon with the topping sugar.
Store in an airtight container at room temperature for up to 2 days, or freeze for longer storage.