Peat Moss

This is the brown spongy stuff sold in bags at the garden center. It might cost a little of your allowance to buy, but it does a good job of helping both clay and sandy soils. One little tip. Be sure that the peat is a little damp before mixing it into the soil. Dry peat acts like a duck's back and water just rolls off it. It is hard to wet it when it's mixed into the soil dry.