To make sure adequate sodium and fluid is obtain, drink a sports drink with a concentration of 20-30 mmol sodium. This should be stated on the label. Note that although most sports drinks contain some sodium, not all contain this higher concentration. The first sports drinks introduced in the 1970's contained high levels of sodium to mimic the composition of sweat. This means that we are less suseptible to dehydration and thus can exercise for longer or at a higher intensity.
Sodium improves palatibility as it offsets the sweetness of the added sugars.
There is also some evidence that after exercise and when dehydrated most people have a preference for a slightly salty tasting beverage Sodium and other electrolytes do need replacing in ultra endurance events over 4 hours. The most common problem arises in those who sweat profusely, as they are at risk of both dehydration and sodium depletion. Many athletes don't drink enough on a day to day basis and often go into events already partially dehydrated. Fluid is absorbed faster if small amounts are drunk regularly. If already dehydrated, fluid will be absorbed slowly and may sit in your stomach or cause a stitch during running.