There is a widespread belief that athletes need a high salt intake. However, sweat actually contains less salt than plasma (the watery part of blood). Therefore, when you sweat, you lose more water than salt and so the levels of sodium in blood actually rise. There is plenty of salt in food and except in extreme circumstances there is unlikely to be any need for salt supplementation.
There is a good discussion of this, including a video on the British Medical Journal website by Timothy Noakes who is an expert in the field. Click here to take a look.