Session Question

Hi there everyone :-) I have a question about Session usage. I am passing data from 1 page to another via the toolbar (Forgot the method name, sorry :-) and then when the client clicks on "Add to shopping cart" it adds it into a session to store the data for checkout or to continue shopping. My question is this: When the client goes back and selects another item, it overrides the original in the session - how do I get sessions to be dynamic and assisn a different value for each shopping cart entry? And then how do I retrieve them? Thanks for your help, I have learnt so much from this group. Chris