multi-page survey - getting user back into it

I need to allow subjects to complete part of a survey one day then finish it the next day. I know it's possible to store the data, then pull them back into wherever they left off, but this may be too difficult for my novice skills. So I'm thinking of just creating one form with a set # of questions they will answer the first day then the rest they'll have to answer the next day on another form. I think I can set a cookie that will auto-increment somehow to use as an identifier (we don't want to have them login or anything). So first, how do I make a cookie variable increment so that none of the users end up with the same ID? Also, I'm thinking I'd just insert data into one table the first day including the ID field that I've assigned in the cookie - actually would appreciate sample of how I pull that id out and insert into the db on the action page. Then on the 2nd day, I'd do the same thing into a different table and just query the 2 tables, joining on the ID field.

Thanks!