Do you have a mailing list? If not, you should be including this as part of your businesses marketing strategy. It's a great way to keep in touch with clients and to keep you in mind. You can even tag people on your list to identify which are existing customers and which are potential clients.
If you are just starting to think about setting up and building mailing list for your business, MailChimp is a wonderful place to start. You can use it to send out out newsletters, promote your free and paid events and best of all, it's free for up to 2,000 subscribers so you really don't have much to loose other than a little time setting it up. It's never too later to start so that you'll be ready to take advantage of it next time you launch an event.
I have even prepared step-by-step instructions on how to set up your first MailChimp email campaign.
By the way, a great way to start a mailing list, especially when you are first starting your business, is to add absolutely everyone you know (current and valid email addresses only) and, for your first email campaign, make it all about letting them know that you've added them to this list, what it's about, how often you intend on sending out the emails and let them know to click the Unsubscribe link if they want to be removed. If they know who you are, most people won't unsubscribe and you'll have established your first mailing list.
IMPORTANT: Only include current and valid email addresses of people you know… you've been warned. Email addresses must be valid because, if too many messages can't be delivered or too many people complain that you are sending spam email to them, MailChimp will disable your account. This has everything to do with MailChimp's acceptable use policy. Other companies have similar anti-spam clauses. Be sure to read them when you are signing up for the service.
Should this happen to you, if you are lucky, you'll be given the option of emptying your mailing list manually and then resend each person a separate email using outlook to invite them to subscribe again.
In essence, the folks who run MailChimp give you the benefit of the doubt to start but, like with any relationship in life, once you loose your credibility and/or trust, it takes a lot of work to get it back.