What is the one piece of advice you would give about attending AXPONA?

You can cover the floors faster if you wear a climbing harness, bring a rope and starting at the top rappel down each floor. ;)

If you have any hearing sensitivities, keep earplugs handy.
I was a newbie at last Oct's RMAF and sat way too long the first
day listening to music played louder than my ears seem to handle.
Consequently my day two and three were not much fun as I my ears
needed time off.

Listen in the hall way. Walk past any room that sounds bad outside, it won't sound better once you are in.

A good sounding room will sound good from several doors down

Leave your wife at home

Three days is a lot of time, so I would use the first day to do a quick survey to find the rooms you are most interested in doing a bit more long-term listening.  Make notes about which room because it is easy to forget or get confused.  The first day is usually not well attended so use that day to visit rooms that you expect will be crowded on other days; you will have more time to talk to the people, and perhaps audition music you bring to the show.  Also use the first day to buy recordings that are for sale because you don't want to look at only left-overs after the best stuff has been picked.

If the sound of a particular system sounds great to you, then you can be somewhat confident that all of the components in that system are at least capable of performing as you like.  If a system sounds bad, don't write off any particular component in the system--all sorts of things can go wrong at a show that does not allow a system to sound its best and you would be cheating yourself to dismiss anything based on this one experience.   

Use the show experience to learn about all sorts of different approaches to achieve good sound and keep an open mind.  There will be plenty of surprises.