if you are using php(I saw username, so I thought it might be validated server-side):
if(whatever){
//code that creates your form
echo "<script type=\"text/javascript\" language=\"javascript\">";
echo "<!--";
echo "document.forms['Form1'].reset();";
echo "document.Form1.username.focus();";
echo "//-->";
echo "</script>";
}

if you are using javascript to make the form:
if(whatever){
//code that creates your form
document.forms['Form1'].reset();
document.Form1.username.focus();
}