Popovers are best eaten soon after baking, while still warm.  If you plan to prepare them in advance, let cool completely before storing in a zip-top bag at room temperature.  Reheat them on a baking sheet at 350F for 5-7 minutes.  You can freeze popovers in a zip-top bag for up to three months. Reheat by placing the popovers directly from the freezer onto a baking sheet and bake at 350F until warmed through, about 8 minutes.