For the flakiest crust, make sure all ingredients (including the flour) are cold before you begin.
SourceMartha Stewart Living, December/January 1996/1997
2 1/2 cups all-purpose flour
1 teaspoon salt
1 cup cold (2 sticks) unsalted butter, cut into 1/2-inch pieces
1/4 to 1/2 cup ice water
Pulse flour and salt in a food processor. Add butter, and pulse until mixture forms coarse crumbs with some larger pieces remaining, about 10 seconds.
With machine running, add ice water in a slow, steady stream just until dough holds together without being wet or sticky, no longer than 30 seconds.
Divide dough in half, and shape into disks. Wrap in plastic; refrigerate at least 1 hour or overnight. Dough can be frozen up to 1 month; thaw in the refrigerator overnight before using.