Sift the flour, sugar, and salt into a large bowl. Add the butter and shortening. Rub the fats between your fingertips and thumbs, or use a pastry blender, until the mixture resembles coarse crumbs. One tablespoon at a time, work in enough ice water just until the dough comes together, being careful not to overmix.
Shape into a smooth ball of dough, flatten into a disk, and wrap in plastic wrap. Refrigerate for at least 30 minutes, and up to two days.
Yield: one 9- or 10-inch piecrust