function clearField(formName, field, value) {
  var fieldEl = document.forms[formName].elements[field];
  if(fieldEl.value == value) {
    fieldEl.value = "";
  }
}

function replaceField(formName, field, value) {
  var fieldEl = document.forms[formName].elements[field];
  if(fieldEl.value == "") {
    fieldEl.value = value;
  }
}