Generally these days I would (budget permitting) send something printed and personalised. We all used to complain about junk mail filling our doormats but now we're bombarded digitally every day I think print gives you more stand out. That said, I'd post, follow up with an email then call. Once they've registered I'd call them a couple of days before the event to check they're coming, know where they're going, how they're going to get there, what time it starts/finishes etc. It can also be an opportunity to suggest they bring a colleague/friend if you're looking to increase numbers/have had some drop out.