I don't know if this is really a hack question, but the best way to store doors is upright (and they take up least room anyway).
The tops should be evenly supported by a wall, or the door could warp.
Depending on what the floor is made of, you must prevent damp rising into the wood. There may be a chance that your shed will leak too, so they should really be stored off the ground.
If there are several doors it will be convenient to stack them together, although they will stack better if you remove the handles (and store them in a bag or box). That way the doors will rest flat against each for full support.
If not, you could stack each door with its handles alternately at the left or the right, and slightly staggered so that the handles are beyond the edges of the door behind it. That might work better if the next pair of doors is stored upside-down, so their handles are at a different height from the pair behind, and will nest better. They can still be stood flat against each other without their handles colliding.
Finally, covering them with a sheet of plastic will help protect them.