[thelist] asp file referring to itself

What's the slickest way to post a form to the same asp file that the form
is on?  

For example, I want one asp file, main.asp.  When you go to main.asp, enter
the info in the input boxes, and hit submit, the page checks to see if a
variable has been set and then displays the proper html for the results. 

If the variable hasn't been set, it goes to the form entry part of the
file.  If the variable has been set then it goes to the results page.  

Does this make sense?  I've done it before but I forget the details.  Any
ideas?