Switching Between HTTP And HTTPS

I wish to have a web site that has most of the pages as normal HTTP pages
but has some areas that use HTTPS. I want to have it that if a user selects
a link to a HTTPS page that they go there an there Session Information is
kept. I also wish to have the pages switch automatically to HTTPS if a
visitor types the URL without the HTTPS. So my questions are:
1) How to redirect to a Secure Page without losing Session Info?
2) How to test if page is accessed by HTTPS, and if not switch to HTTPS
quickly?

I think something like this code is what I want but how do I do it.

<%
if not "HTTPS" then
response.redirect(https://mysite.com/securepage.asp)
end if
%>