It's something of a nuclear option, but gargling with hydrogen peroxide is permissible. As a radical preventive measure, it's pretty effective, I'd say; I've used a 50% peroxide solution* twice over the past seven days to keep a sore throat from turning into a sorer throat.
The problem with using hydrogen peroxide as a gargle is that it kills everything. A bit of internet research quickly reveals that overusing peroxide can actually result in worse infections: the solution makes no distinction between beneficial and harmful microorganisms, killing good and bad mouth flora alike. With the good flora gone, i.e., the flora that plays a role in defending you against the bad flora, you're open to invasion by even worse than the usual baddies currently inhabiting your oral cavity.
So I use peroxide sparingly. Two hits were enough to drive back the demons last week, and I haven't had a sore throat since.
Oh, and be sure never to swallow hydrogen peroxide. It's poisonous.
*To clarify: household peroxide is already a solution, usually about 3-6%. By no means am I putting pure peroxide in my mouth.