Why do I talk about sodium?  Sodium is the element that you will lose the most in sweat.  There are high amounts of Potassium is in a lot of sports products for some reason, but the ratio from sweat loss is about 90/10 of Sodium/Potassium. 

You can tell when you get far behind on sodium intake by these indicators: swelling of the hands (and feet), water sloshing in your stomach, calf cramps (or any other muscle cramps), and nausea.  The swelling of the hands and feet factor has a lot to do with runners getting blisters on their feet.  I normally don't get blisters anymore, even in 100-milers, due to having the electrolyte issue "dialed-in" better.  Sloshing in your stomach is caused by your body not letting in any more low-sodium fluid or food, which would dilute your blood electrolyte levels even more.  And muscle cramps are caused almost exclusively by low electrolyte levels.

Sometimes you will have no indication of a low electrolyte balance during the race, but the next morning after the race, you'll have swelling and stiffness.  This can make traveling back from an event problematic, if you have to drive or fly long distances.

My favorite brand of electolyte is SUCCEED, which you can get locally at Gribbles on 119th St & Quivira.  Endurolyte (brand) from Hammer Nutrition are fine to take occasionally, but in ultra events lasting 8 hours or more, their high potassium to sodium level can be problematic for some runners.  They also don't have as much sodium in them, so you have to take (and buy) more to get the same effect as with the SUCCEED brand, so there's not as much "bang for your buck."  I usually use this strategy in a 100-miler:  I take 3 Succeed capsules for every 1 Endurolyte (over time).  Yes, I mix them.  Endurolytes have more magnesium as "trace minerals" in them than Succeed, and the magnesium comes in handy late in a 50 or 100-miler.  Again, this is what myself (and other ultrarunners and ultracyclists) have gleaned from experience, and from exchanging our successful strategies.