How can I stiffen the subfloor of an upstairs bathroom?
There are two choices to stiffen a floor system


Halve joist span with a beam.
The beam-in-joist method has the advantage of minimal final impact on the room below.  It does mean spanning the beam to a load bearing walls.
Double up joists in area.
This requires removing floor or ceiling to sister existing joists. The joists also have to span as far as the existing joists.


The beam-in-joist solution should be confirmed with a building plan review for point load considerations under the transferred supports.   

Update: I did come across a very thorough article from Fine Homebuilding "6 Ways to Stiffen a Bouncy Floor". (its a .pdf file)

Could I suggest a roll or 2 of Ditra (Schluter) under some nice tile?