Clavamox to start, then Zithromax or Doxycycline if that doesn't work. Lysine every day for maintenance.
The newest research indicates that many URIs are caused by mycoplasma, not bacteria or viruses. Zithromax and Doxycycline are both effective against mycoplasma as well as bacteria so they pretty much cover all your bases. I just use Clavamox first because it has less risk of side effects.