How do I keep this polyurethaned oak floor in good condition?
The thing that deteriorates polyurethane most quickly is allowing abrasives (like sand) to accumulate the floor.  These should be swept up immediately; or just set your roomba to do it for you.

A swiffer, or hard wood floor cleaner such as this between 1ce a week - 1ce a month depending on how heavy traffic is should be fine.  

Steam mopping is a bit questionable, but probably fine for finishes that are in good condition.

Sand and refinish the floors every 5 years. 



Other things I've seen deteroiate floors is


soakings 
heavy moppings
the use of bleach
the use of boiling water (not to be confused with steam mopping)