I set up an email automation campaign one month or more before the event. My automation will include a series of 'if/not' decision trees; If my prospect opens and clicks on the first ever email I send them one which is more personalised and has more detail in it.
If not, I send them an email that will try to capture their interest with a catchy subject line and a variety of topics. If my prospect hits on the event landing page but does not sign up, I will send them a reminder email a few days later, again, perhaps with some video of a previous keynote and so on.
I create an excellent Landing page to maximise the number of attendees.