"Use Twitter to send out a tweet from your organization offering a registration discount to the conference valid or the next 36 hours only. The offer is available from that exact moment but won't last. Tell them to register right now and send it on to their colleagues as time is ticking away. If you have the option of using coupon codes in your registration system, you might also give a prize for the person referring the most registrants during that time frame."
Give it a try. What do you have to lose?