The APUs seem to provide enough power for the electric demands, however, there's just not enough "bleed air" (compressor-supplied) to provide any sort of comfortable cooling. Upon disembarking, the cabin crew usually requests passengers to close window shades prior to vacating their seats...some do, many don't.

The best thing you, as an arriving passenger, can do is, upon taking your seat, put the shades down. If you board on Zone 1, or 1st class, more than likely there won't be people immediately behind you. That being the case, you would probably do well to flick the shades down before stowing carry-ons...those few extra moments can make a big difference in air temp.