Drywall (sheetrock) won't necessarily help your soil. Its main ingredient is gypsum, and gypsum is used effectively to amend soils high in sodium. Gypsum needs to be ground to a fine powder to react with the soil. Unless your clay soil has a lot of salt in it, though, the gypsum will be of little use in improving the soil. That said, the best way to deal with clay is to amend it with lots of organic matter, such as compost, peat moss, shredded leaves, straw, composted sawdust, or well-rotted manure.