1) A guide at the Black Country Museum (Dudley - owr kid) said

"Tea or coffee but nothing with milk in it" and he was certain that the milk would affect the voice over his shift of maybe 8 hours talking.

2) And Paul Metsers, the Kiwi singer living in Cumbria, reckoned:

"Water with just a hint of lemon juice" would preserve his voice.

3) I read in the New Scientist that your recall is best in the conditions you learned. Which is why I prefer cider**, even at dances/folk clubs/session where there is no alcohol on offer.

**other nectars are available