Sessions and recordset

I am still having a bit of problem trying to create a recently viewed items application.

I find <?php session_start();> should be placed at very start of pages requiring session variable.

I want to store Product IDs that will be generated by visitor when they view product details page

Then I want to use these ids to be displayed in a recordset called rsRecentlyViewed.

I can't seem to store a visitors session i.e. items with specific ids so a recordset can filter and display thumbnail, title and price in a particular div from db.

What do I do next ?

Also another question along same lines is how can more than one session variable be made available for receiving page to use? I am also attempting to create an upselling feature as well on same page in regards to this (can you have more than one recordset on a page).

Thanks