Seasonal decorations are an unusual storage category: used for a fortnight, stored for fifty weeks, and handled by someone who by then has forgotten entirely how they were packed. That combination is why most households own three sets of tangled lights and two boxes of things they never put up.
Nearly all of the difficulty is created on the way in, during the tired job of packing it all away, and that is where it is cheaply fixed.
Thin it out while it is still up
The only moment you can see what actually got used is while the decorations are in place. Anything still in the box on Boxing Day did not make the cut this year and very likely will not next year either. Deciding then takes a minute per item; deciding in eleven months' time takes far longer and goes the other way, because by then everything looks potentially useful.
Pack by room, not by type
Most homes decorate the same few places every year. A box labelled "front room and hall" is unpacked in one go and put straight up. A box labelled "baubles" has to be distributed around the house and sorted on the floor first, which is how December evenings disappear.
The exception is lights, which deserve their own container because they are the thing most likely to fail and most annoying to hunt for.
Lights need a core, not a bag
Wound round a piece of stiff card or an offcut of pipe, with the plug taped to the card, a set of lights comes out usable in ten seconds. Loose in a bag they emerge as a single knot every single year. It is a two-minute job in January that saves half an hour in December, which is about the best trade available in household storage.
Test before storing, not after
Plug each set in on the way into the box and write the result on the card. A dead set discovered in January is a thing you can replace in the sales or bin without regret. The same set discovered on the first weekend of December is a trip out on the worst shopping day of the year.
Fragile things want rigid boxes and dividers
Glass decorations survive in something with compartments: an egg box, a wine carton insert, or a shallow rigid box with a tea towel between the layers. Soft bags crush, and a loft crushes more than a cupboard does because things get stacked on top of them in July by somebody who has no idea what is in there.
Store cool and dry, which is rarely the loft
Lofts swing from freezing to very warm and that is hard on candles, chocolate decorations, adhesive tape and anything with a battery. A cupboard, the bottom of a wardrobe or under a bed is kinder. If it has to be the loft, keep the heat-sensitive things out and put them in a single small box indoors.
Label for a tired person
Write on the end that faces out when the box is stacked, in large letters, with the year. The year matters: it tells next year's version of you when the contents were last thinned, which is the information you will actually want.