The trick to the piece not breaking is mainly to prevent twisting forces and sudden release of tension, of the kind you get when you cut through the sprue attachment points with a knife or cutters. For that reason, I normally saw fragile parts loose instead. Use a fine-blade razor saw, or better yet, a razorblade saw (a razorblade shape with teeth cut into the sides that are normally sharp) and make careful cuts through the thin part of the sprue close to — but not touching — the part. Then clean up with a sharp knife, file, sandpaper, or whatever you like.