In reply to EricaR:
Some folk sweat out a lot more salt than others. So as others have said you may need to replace your Salt/electrolytes not just the water. It's and individual thing though, so try it and see what works for you. If I'm out in the sun jogging/walking/climbing and sweating all day then I tend to alternate one bottle of sugary water with one bottle of water that has a electrolyte tablet added.
Conditioning also makes a vast difference in my experience. Having surfed every swell of our uk winters for 35 years before coming back to walking and climbing. I find I am coping better with the heat of dry land each year, despite getting older a probably less fit.
The other thing that may help is local massage, when the muscle is feeling fatigued, especially if your cooling down. I'm no physio but I think it must help relax the muscle tissues, works for me any way, but we are all different.