Determine how users will enter and exit your catalog. There are quite complex and intelligent conditional schemes possible, especially if you use the Cookies capability, but simplicity is often the easiest and most reliable.
The most important thing to remember is that if you are supporting browsers which might not accept cookies, you must never send the user to a page that is not served by MiniVend. If you do, they will lose their session (and items in their shopping cart).
If you are using MiniVend's frames mode, you must be careful to source all frame panes from a page served by MiniVend. If you do not, then you may find the user has multiple session IDs depending on which frame the link came from.