How do I remove a sheet cake from a pan?
Let the cakes cool in the pan for 15 minutes. 
Carefully run a table knife around the edge of the pan to make sure the edges of the cake are fully released from the edges of the pan.
Place a wire rack over the cake pan, then invert the rack and the pan. 
Pull the pan off of the cake. Let the cake cool to room temperature.
Invert the cake again onto some plastic-wrapped cardboard. 
Move the layers on the cardboard to your freezer for at least 2 hours.  We usually bake the cake the day before and let them freeze overnight.  We always had an upright freezer in the garage, and 2 layers would fit next to each other on one shelf.


When you pull the cake layers from the freezer, they'll be solid and reasonably sturdy, but you should still exercise care, using both hands to support them.  It's perfectly fine to assemble and ice the frozen cake layers.  I've never had a problem with them sweating as they thaw.  But, there's no harm in letting them thaw a bit after assembling if you're worried.

Bonus: there are fewer crumbs when you ice a frozen cake.