I've seen some people lay strand on their sides and coil/curl them up and place in a tub(s), separating each layer/string with a piece of cardboard or similar.
Others lay the strips flat and suspend from ceiling of garage.
Might depend on what material you are using for your strings in the first place.
I just do a messy job of the first suggestion, and regret it each November!
I've dedicated a wall in the garage to store them, built a long shelf so they can be stored straight and as flat as possible. Will see how well it works this weekend when I take the tree down
I put mine in some garbage bins. Easy to lay in there, and not too heavy to lift. They were just some Aussie made ones. I also put all my extension cables in one too, help to keep the rolled, and not tangled and snagged on eachother.
This method was fine, but I wanted a way to test my pixels prior to installing them the following year, I did get them all out of the storage box and test etc which was ok.
I then come up with this little idea as a prototype "Tie Fighter" Mega Tree strip holder.
I was able to simply connect a pixel tester to each string in preparation to setting up as the 3mm coro allows you to see if the pixels are all working without having to unwind the string.
Yes this storage solution is totally not needed, but I just wanted to try a alternative Mega Tree pixel storage idea.