Sometimes we tend to go a little overboard when a case (some are very pretty) containing a large collection of ME thread or sewing thread goes on sale. Even single spools can entice us to fill up the shopping cart, and sales are a good thing. 
Now, if you sew or embroider daily, stitch large designs/projects or have a business the collection can dwindle. However, some of us start out strong with good intentions and then life's precedence takes a turn and our interests (instead of the thread supply) dwindles! 
But, you want to take care of your expensive investment so it does not have to be replaced when you get your groove back. 
If you live in a dry climate, thread can become brittle. Same for exposure to sunlight*. And, unless you live in a bubble, household dust can collect on the outer layer of the spool.
Air-tight storage can help eliminate drying out (or, put in baggy then place in fridge crisper** if there is room) do same for wss stabilizer. I happen to have my work area and supplies in my basement, which is a little more humid than the rest of the house.
Craft storage units can also help protect thread from sunlight and dust.
Peel off the external layer of thread on spools that have already been stored in the open, then place where it will be dust free.
I test my thread by giving it the snap test to make sure it is not brittle before I use it.
More suggestions and tips welcomed!
*There is a type of ME thread that does not fade in the sun. I don't have any of this, but would store it the same way.
** Unless the manufacturer suggests otherwise.