A good way to treat cramps from sweating includes taking sports beverage with electrolytes. You need also to stretch and massage the cramping muscle or hold the joint in a stretched position until the cramp stops. If the cramping does not stop in an hour, seek medical attention. Read http://sportsmedicine.about.com/od/enviromentalissues/a/Heat-Cramps.htm.
John, if you sweat regularly and heavily, check your general health. If you are talking about sweating after sports, it is probably to do with your electrolyte level. Soldiers sent to hot desert countries are usually given salt tablets to take each day, for the same reason. Try increasing the salt in your diet, even having a teaspoonful of salt in water before sports, and see if that helps. If not, see the doctor.