Use testimonials from previous events
Testimonials can be very powerful if used in the right way and can be applied to any sales situation including selling an event. The most effective testimonials are short, and contain specific details about the event.

Partner with another company to get discounts to attractions, services, etc
Offer your event participant’s discounts to attractions or services may provide extra incentive to attend.

Partner with a related but non-competing event to promote each other’s event
If you know of a related but non-competing event contact the organizer of the other event to see if there is an opportunity for cross promotion. Maybe you can swap flyer inserts, mention each other on the event websites or get a mention at the event. This idea can expose you to a whole new market, won't cost anything and will benefit both events.

Include an e-book or whitepaper with registration
Giving something of value away for free to your event participants doesn't have to mean spending money. E-books or whitepapers can be very valuable and once you have permission to use one, it is free to send via email. If you don't already have a document to send, find one on the internet by doing a search for "ebooks". Contact the author and tell them how you want to use the document and you may get permission to use it, especially if it will help promote the author's business.

Ask people to enter the name of another person who may be interested in the event
If someone is registering for an event they likely have a colleague or friend who would also be interested in attending. Most of the time people just don't think of suggesting it to others, so you should remind them. Something as simple as text on the form or in the confirmation email can be enough to get people to act.

Posted - Matt Allely. (Article by Jim Romanik)