So, you've probably already packed up your lights for the year, but keep this trick in the back of your mind for next year.
I don't know what made me try this in the first place 15 years ago, but now I would never store strings of lights any other way.
In 15 years, I have never had a string of lights tangle, and I can find the specific string I'm looking for the first time, every time. This system cost me $2.99. Once. 15 years ago.
Ready for the secret? Lunch-sized paper sacks. Buy one package of them and keep them with your holiday decorations. You'll still have some left over in a few decades.
Put each strand of lights in its own paper sack. Feed the lights in like you're stuffing a sleeping bag into a stuff-sack, don't roll them up or fold them or bunch them first. This is part of the secret. Start with the non-plug end at the bottom of the sack, so the plug always comes out first.
Need to test a light strand? No need to remove it from the sack, just pull out the plug end a few inches and plug it in. Don't worry about a fire hazard, the lit lights won't be in the sack long enough to matter. In 15 years, I've never had a sack even start to get brownish from lit lights.
Another brilliant aspect of this system is easy labeling. I always include on my sacks:
1. Number of lights in the strand
2. Color of bulbs
3. Color of cord (green looks good on the tree, white looks good against white walls)
4. "Plugs one end" or "Plugs both ends" for decoration planning purposes
5. Any other distinguishing characteristics (like "LED" or "icicle style" or "faded color" or even how you typically use them, like "goes above the fireplace")
You can fit a remarkable number of these paper sacks full of lights into a large plastic tub, with other holiday decorations or by themselves. The sacks are also conveniently squishy, so you can really pack them in to any available space.
Next year when you pull them out, you'll know exactly what you've got without even opening any sacks. Throw away (or recycle) all those stupid plastic clips and cardboard boxes the lights came in but never fit back into. One flimsy paper sack will last for many years (I use the same ones over and over again. I've replaced a few over the years, but not many; the sacks tend to last longer than the lights themselves).
One more bonus: decorating the tree is easier. Pull out the plug end and plug it in. Keep the sack full of lights in your hand as you walk around the tree, feeding out only as much of the strand as you need for that part of the tree. The tail end of the strand won't get caught on lower branches, or, you know, the cat. Work in reverse to take the lights down.