Upgrading out of Houston is very tough.
I would say forget about day of departure
upgrades. There are enough people paying
fares that are upgradable in advance that you
stand very little shot at a standby. I have
sometimes connected thru Houston and even
getting checked in at one of the coasts, the
out-of-Houston upgrades are the toughest.
Don't forget to look at America West flights,
upgrade privileges are identical.