Reduce Size of SYSAUX
Try moving some objects into another tablespace. 
Documentation on this is quite confusing, referring to an uncertain pl/sql procedure, that should be used. Though, there is no mention what procedure it is. I would use alter table ... move ...

Edit: (thanks to @Jack Douglas):
Instead of alter, there is a move_procedure defined in V$SYSAUX_OCCUPANTS that should be used to move those occupants.