чуть доделал, чтоб в минус не уходил
<!DOCTYPE html>
<html>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=windows-1251' />
<script src='https://code.jquery.com/jquery-latest.js'></script>
<style type='text/css'>
</style>
<script type='text/javascript'>
$(function(){
$('.number > button').click(function(){
var o=$(this), c = o.parent().children('.amount');
var i=(o.hasClass('inc'))? 1:c.val()!=0? -1:0;
o=c;
o.val(parseInt(o.val())+i);
});
});
</script>
</head>
<body>
<div class='number'>
<button class="dec">-</button>
<input class="amount" value="1"/>
<button class="inc">+</button>
</div>
<div class='number'>
<button class="dec">-</button>
<input class="amount" value="1"/>
<button class="inc">+</button>
</div>
<div class='number'>
<button class="dec">-</button>
<input class="amount" value="1"/>
<button class="inc">+</button>
</div>
</body>
</html>