Old kits? I do either, or a combination, of the following:
1. The kit is "put to grass." That is, displayed out of the protected display cabinet.
2. If I plan on building another one, I put it in one of my "Davis Monthan" shelves along with unfinished projects. I could strip them of spare parts later on, or, use them as testbeds for experimental techniques.
3. I sell, trade or give it to friends who might be interested.
4. For rare kits or those with sentimental value I disassemble very carefully and start a restoration project.