Q1247 How can I prevent the user from checking more than x checkboxes

<script language="JavaScript"><!--
maxBoxes = 3;
boxesChecked = 0;
function checkIt(theBox) {
   if (boxesChecked+1 > maxBoxes) {
      alert('Max '+maxBoxes);
      return false;
   }
   boxesChecked++
   return true;
} 
//--></script>

<form>
<input type="checkbox" onClick="if (this.checked) return checkIt(this); else boxesChecked--">
<input type="checkbox" onClick="if (this.checked) return checkIt(this); else boxesChecked--">
<input type="checkbox" onClick="if (this.checked) return checkIt(this); else boxesChecked--">
<input type="checkbox" onClick="if (this.checked) return checkIt(this); else boxesChecked--">
</form>