A couple of things I've been doing:

When kids have a birthday or holiday with gifts, we tell them that they have to throw away or give away an equal amount of stuff. It works surprisingly well.

I've been thinning out my media stuff (books, videos, old records, etc.) in batches during moves, but recently began selling what I could on amazon. It's mostly CDs and books, but in the past two months it's totaled about $800. One problem, however, is many titles are worth too little for me to go through the trouble of listing, selling, packing, and shipping them. This is especially true of older books. No matter how good or important they were, about 90% sell for <$2 used on Amazon.