What is the best way to maintain and make secure session? Code I use at the moment

Session start after auth:

session_regenerate_id();
        $_SESSION['loggedin'] = TRUE;
        $_SESSION['name'] = $_POST['username'];
        $_SESSION['id'] = $id;
        header("location: main.php");

And then on every page i use:

session_start();
// If the user is not logged in redirect to the login page
if (!isset($_SESSION['loggedin'])) {
    header('Location: index.php');
    exit;
}

How to improve security over all question. Also how to improve it without SSL