Frames help needed

I'm creating a site that uses frames (1 on the left side of the page & the main
frame). The site shows other sites 1 site at a time, kind of like a web ring,
the sites are shown in the main frame. In the left frame, I have 3 links: 1.
Back to my site, 2. Next site, & 3. Previous site. My problem is that I can't
get the main frame to show the previous site, when the user clicks on the
"previous site" link in the left frame.