Out of Nancy Clark's "Sports Nutrition Guidebook":
"For a low-cost fluid replacer that has a nutritional profile similar to the more expensive commercial sports drinks, try this simple recipe."

4 tbsp sugar
1/4 tsp salt
1/4 c. boiling water
1/4 c. orange juice (not concentrate) or 2 Tbsp lemon juice
3-3 3/4 c. cold water

1. In the bottom of a pitcher, dissolve the sugar and salt in the hot water.
2. Add the juice and the remaining water; chill.
3. Quench that thirst!