The question is a good one! I restore old or ill cared for tack as a business. I've owned equines and there associated "outfits" for 45 years. I've ruined tack and have also been able to preserve a 45 y/o halter in pristine condition. If you want your stuff to last and look good next year, it takes just a little forethought and effort.
The first question to ask yourself is, "What is my environment like and how does it change through the seasons?" I live in the Sierra Foothills in CA, cold
low humidity winters and blistering hot and dry summers, where humidity isn't really a problem, but I also lived in Hawaii, muggy and damp everyday of the year, where leather tack would start molding in two weeks!
The easiest remedy for preserving tack is controlling the storage environment. First, if you can store items indoors..ie. in a house 1st, attached garage 2nd or enclosed tackroom 3rd, your tack will stay in better condition as it won't have to endure the temp and humidity fluxuations, and the problems that come with the changes, as it would in a open stall or barn. Know that it will come out of storage in a condition that is based on Awesome, OK and O Well based on 1st, second and 3rd noted above!!
Second, clean and condition the tack before you store it, so it will have a better chance of not "catching some malady" ie. mold, rust and corrosion, heat discoloration, permanent "leather death" (meaning too dry to bring back to safely useful) before you want to use it again. Make sure all leather is rolled or layed out in the way you would want it to be when you get it out again. ie...roll up reins and headstalls so that the out is out and the in is in. Leather has a "lay" to it, and you don't want to store it "against the lay". Once your start using it again it will go back to the shape you expect without resembling "a bad hair day"!
I wrap or roll several similar items up in old rice bran bags (the ones that are woven plastic with no liner, because they breath) and lay them in a plastic, lidded bin in the coolest, least indirect sun/heat exposure corner on the floor of my garage. I have no room in my air conditioned house. If the humidity is high in the spring or fall, I crack or open the lid fully. Leather is/was a living thing, and just like properly storing fresh veggies, fruit and meats in the frig, how you store it, effects how long it will keep.
JMHO...
Sandi