I have one thing to add. When sending any data that is persistent (like a cookie) to the client, you should ALWAYS include a hash (like MD5 or SHA1) of the original value so you can easily see if the cookie value has been modified.
Also, you should use cryptographically sound session ids to lessen the chance of guessing a valid session. I came across a very good paper recently that talked about this. The URL is http://www.usenix.org/events/sec01/fu/fu_html/index.html.
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