The Private mode of your browser is a good idea.
You also can create cookie-less landing page templates and create a landing page from this template. See this thread : Non-trackable landing page is still identifying me . The issue is that this technique will erase also your own cookie.
Another, simple solution, if you can setup the DNS is to have your LP hosted in a separate domain.
Finally, you can also vote here Create true "Untracked" landing pages
The incognito window is the simplest solution. If you're willing to have a separate registration page for each event, though, then you can use the code from my blog here to clear the tracking cookie submission and prevent the bad cookie association without a new incognito window: http://developers.marketo.com/blog/clearing-marketo-tracking-cookie-from-forms-2-0-submission/
Thanks Kenny. I will send this code along to my developer.