How often do we need a reel of cotton, only to find a tangled mess of cottons in our sewing box, or reels that have become dust gatherers as they sit on our reel holder.
My way to overcome this is to keep each reel in a little zip lock bag with just the start of the thread sticking out. Now when you need to use a particular colour, just pull your thread to desired length and cut, leaving the start of thread ready for the next time you need to use. No more tangled threads, no dusty reels and no need to open zip lock bags (until you need to replace your empty reel)
Of course if you have the space (oh that we should all be so lucky)you can put a peg board up and punch a whole in the top of zip lock bag and hang your cottons up for easy identification.