I used to work in really hot conditions, the medical advice was to avoid tea, coffee or anything else containing caffeine, eat raisins, drink more water than you think you need (without being daft obviously). Also your body starts to produce a chemical which helps with the heat, but not until you've been in the heat for about 4 days, which I know isn't much help. Basically be sensible I found it does get easier the longer it goes on.