как вариант:
<form name="form">
<input type="button" value="up" />
<b id="b"> 0 </b>
<input type="button" value="dn" />
<form>
<script>
window.onload = function() {
var form = document.form.elements, i = form.length, j = 0;
while(i--) {
form[i].onclick = function(i) {
return function() {
document.getElementById('b').innerHTML = i ? ++j : --j;
};
}(i);
}
};
</script>
допилите сами