Allowing user to return and finish survey later

I am writing a web-based survey and would like to allow users to log off and return later to finish the survey. I believe that I should use cookies but have no idea how to go about it.

How can I allow users to return to a survey at a later point in time without losing their data from the previous visit (on the same form). (The survey will have multiple pages.)

Thanks.

oh ... this is related to the other post ...

Yes, you'll need cookies on this one, but if someone clears their
cookies automatically, they can't come back. So, you'll have to
assume they have them enabled.

At this point, it would be best to have a link to your survey so we
can see the variables you are using.