You could go 2 down, 2 up, attach w/2x4's and then add on sections as necessary. I use the 2x4's as I can't drive a stake straight for the life of me!
Here's a pic I've posted before, really should get something more current to show my progress:
For winter I am adding pallets to the tops supported by 2x4 runners and then will add tarps to the top pallets. By doing it this way I don't have to deal with sagging, snow & ice filled tarps
I have a couple of rows going like this with space between the rows. Having the space between rows like this I don't have to walk on the pallets to retrieve wood as I can access either side of the stack.
Shari