Don't store it in the sun.
Cover it with a tarp if you have to leave it outside.
Stack it neatly. The weight of the upper planks keep the lower planks straighter.
Choose kiln dried if available.
If you're going to store it for a long period, sticker it so all four sides of each plank are exposed to the same conditions (primarily temperature and humidity). "Stickers are small pieces of wood (usually about 3/4” x 11/2”) that are used to add space between sawn planks, which increases ventilation and aids in a more uniform drying process. Sticker spacing varies depending on the species and thickness of the lumber being dried; a conservative spacing scheme would be every 12”, though usually 16” or 24” spacing can be safely used on thicker pieces."
Here's a good article:
Drying Wood at Home | The Wood Database