在文本框中显示 请输入什么信息

<script type="text/javascript" language="javascript">
var txtTrain=document.getElementById("txtXianlu");
function searchclearInfoJs(obj){
if(txtTrain.value=="输入地点或车次" && obj==0)
{
txtTrain.style.color="#000";
txtTrain.value="";
}
}
function searchinputInfoJs(obj){
if(txtTrain.value=="" && obj==0){
txtTrain.style.color="#999";
txtTrain.value="输入地点或车次";
}
}
if(txtTrain!=undefined){
if(txtTrain.value==""||txtTrain.value=="输入地点或车次")
{
txtTrain.style.color="#999";
txtTrain.value="输入地点或车次";
txtTrain.onfocus=function(){
searchclearInfoJs(0);
}
txtTrain.onblur=function(){
searchinputInfoJs(0);
}
}
}
</script>

在form后面写