Try <?php echo "$username"; ?> or whateber your input names are..
Or you could make a cookie for each of the field names and then echo this cookie for the input values.
on submit..
setcookie('username',$username,time()+600);
if things are wrong once submitted and u send them back.
<input type="text" value="<?php echo $_cookie['username']; ?>">
i think the term for this is "making forms sticky" you could have a scan around u might find something.