I don't know that there is a way to preserve them, other than to keep them in a cool, dry place (like the kit box). Many of my stash kits are also getting up there in age. Some in the 10-15 year range. If I start one, I will use my scanner to make a duplicate image of the decals. This way I can print them on home decal paper if there is a problem with the original sheet. Also, when I start the decaling process, I will cut one of the smaller, not readily seen decals out and do a test application. This can help you to judge the condition of the sheet, although usually I've found that the bigger sized decals are the ones that give you trouble. Oftentimes decals sheets have several marking versions on them, so you can pick ones you don't want on the model and use those to test, also.
Hope this helps.
Chris