<script>
function arttir(input){
    input.value = parseInt(input.value)+1;
}

function azalt(input){
    var eskiDeger = input.value,
         yeniDeger = parseInt(input.value)-1;
    input.value = yeniDeger < 0 ? eskiDeger : yeniDeger;
}
</script>

<form name="form">   
   <input type="button" value="-" onclick="azalt(document.forms['form'].elements['sayi'])" style="width:20px;font-family:arial;font-size:12">
   <input type="text" name="sayi" value="0" style="width:20px;font-family:arial;font-size:10">
   <input type="button" value="+" onclick="arttir(document.forms['form'].elements['sayi'])" style="width:20px;font-family:arial;font-size:12">
</form>