Protecting webpage from remote submitting

I want to know what is quality approach to stop remote submitting a page. For example I have an ASP page submit_form.asp that use HTML FORM element and this form is submitted to process_form.asp page. What is best way to stop users from sending requests to process_form.asp page from remote servers.

You could check the referrer, I suppose.  If they submitted the form from your website, wouldn't your website be the referrer?  If the referrer is not what you are expecting, then throw an error message and not process the request.

It might be interesting to take a look at how Borg does it.  He has several scripts that should not be called directly, such as the one that adds votes to a poll.  I know that if you try to access it directly in the browser (i.e. its not called by pressing the submit button on the poll's form), it will not count your vote and redirect you to the forum's default page.  It only works if its called by the poll's form.  (I tested to make sure people couldn't cheat in the Battle of the Islands competition we have.)  I've never disected the file, but I'm sure Borg did a good job at preventing direct submissions, all without using CAPTCHA for voting in the poll.