A little bit of randomness. We had an air conditioning tune up yesterday. Basically, they just checked everything out...no actual maintenance was required). During the check-out, they asked if I had a bunch of vents closed. I had been closing the downstairs vents to force the cold air upstairs in the summer. However, the idea backfired somewhat, as this just created back pressure that kept the cold air sitting in the vents rather than coming through the house.
The technician told me never to close more than 1 or 2 vents in the house at any given time. Another AC tip - to reduce the temperature difference between upstairs and downstairs (without zoning), set the fan to "ON" rather than auto. The AC compressor won't kick in unless the temp gets to high, but leaving the fan on will ensure that the air continues to get circulated throughout the house.