This issue comes up quite a lot URL encoding or cookies seem to be the most popular choices. However I have little experience of this so I will leave it to the experts to answer your question:

• Login Solutions crazyinsomniac's node is particularly good
• merlyn's as every excellent response is here
• Encrypting or hiding certain info in a URL
• unique session id Off topic but may answer your next question once you take a gander at the other threads ;-)

HTH
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Zigster