For practice I use scraps of whatever batting I have on hand (usually Warm & Nat. or Hobbs Heirloom Fusible). I often use muslin and felt to try out a new design. Having said that, I always do a warm-up before tackling any quilt, using the same backing and batting I'm going to be using in the actual quilt. My favorite batting is White Rose or Cream Rose by Mountain Mist, I love the softness and it has such a nice drape. It is harder to find and more pricey so I usually default to Warm & Nat or Warm & White.