What do I need to store in the php session when user logged in?

Currently when user logged in, i created 2 sessions.

$_SESSION['logged_in'] = 1;
$_SESSION['username']  = $username; // user's name

So that, those page which requires logged in, i just do this:

if(isset($_SESSION['logged_id'])){
// Do whatever I want
}

Is there any security loopholes? I mean, is it easy to hack my session? How does people hack session? and how do I prevent it??

EDIT:

Just found this:

http://www.xrvel.com/post/353/programming/make-a-secure-session-login-script

http://net.tutsplus.com/tutorials/php/secure-your-forms-with-form-keys/

Just found the links, are those methods good enough?? Please give your opinions. I still have not get the best answer yet.