They used to have salt-tablet dispensers next to the drinking fountains in factories where it got really hot. I'd do one or two every couple hours, but I honestly have no idea if they did any good. You need a certain amount of potassium and sodium available for the ion channels in your cells. Those are critical.
Gatorade has these covered with sodium citrate and monopotassium phosphate; they throw in some sugar for a feel-good energy boost.
Adequate plain water and sensible food will have you covered.