Somewhere dry and clean, in separator bags, and locked up if necessary. Silicone composites shouldn't touch each other, or touch pure silicone. Glass and metal shouldn't be clanking against each other. Jelly should be kept separate so it doesn't stink up the whole collection, or suddenly decide to melt all over everything. Cyberskin type materials need to be stored with corn starch.
In my opinion, anything porous should be stored in something like a Sugar Sak, which is treated with antibacterial chemicals.
Batteries should be taken out of things with batteries, and everything should always be cleaned before it's put away.
There are storage containers on EF, or you can use a toolbox, a plastic shoebox, or whatever works for you.