For usability purposes I like to set up my form fields this way:

<?php

$username = $_POST['username'];
$message  = $_POST['message'];

?>

<input type="text" name="username" value="<?php echo $username; ?>" />

<textarea name="message"><?php echo $message; ?></textarea>

This way if the user fails validation, the form input he entered previously will still be there and there would be no need to start from scratch.

My problem is I can't seem to keep check boxes selected with the option that the user had chosen before (when the page refreshes after validation fails). How to do this?