Like any cake, a half sheet cake should be cooled properly before you attempt to remove it from the pan. This gives the center time to set so the cake doesn't crumble. Without proper cooling, a cake that measures 11 by 16 inches and serves as many as 40 people is sure to fall apart, leaving you scrambling to "glue" it back together with cake frosting. Bake your cake a day before you wish to decorate it for the best results.
When the cake is finished baking, remove the half sheet pan from the oven and place it on a wire cooling rack on the counter. If you can't balance the large pan on a single cooling rack, place two cooling racks side by side.
Leave the cake on the wire cooling rack for about 20 minutes.
Transfer the pan to the refrigerator and chill until the pan and the top of the cake feel cool to the touch, which may take another 20 minutes. Leave the cake uncovered during this time; covering the cake can produce condensation on the plastic wrap that could drip onto the cake and make it soggy.
Wrap the cake and pan with plastic cling wrap to keep it from drying out. Place it in the refrigerator for one to two hours more so it cools completely to the center.
Slide a rubber spatula or knife around the cake and walls of the cake pan to loosen it. Place a cake board, cutting board or similar object on top of the cake pan. Hold the two tightly together and flip it over so the cake pan is on top. Set the cake pan and board on the counter. Tap the cake pan on all sides to help loosen the cake. Lift the cake pan away slowly. If the cake sticks, insert a spatula or knife to help free it. Decorate the cake as desired.