I'd say don't smoke for about 2 days prior to your test. Make sure you drink lots of water, niacin helps get it out of your body, and just brush your teeth a lot.
Take a tube of Bengay, and smear some directly on your genitals and anus. This may sound strange, or outright bizzare, but the cooling additive in Bengay, Icy Hot, and similar cremes has been shown...