I assume that you know basic details for target speakers such as what topics they have been speaking on, in what kind of conferences, and where they have been often travelling. Share your conference details for
(a) the kind of audience you expect
(b) the conference topics (multiple tracks if you have)
(c) the names of other confirmed speakers
(d) whatever program if finalized even if without the actual schedule
(e) the business and networking opportunities

Share the buzz and how the community is active and involved in your conference. This gives them a real behind-the-scenes picture your planning and vision of the conference.

Next, pick some of their tweets or blog posts (or their comments, notes) that talk about their related experience and skills, and write a note on how they can add real value to the conference.
Last, offer them some incentive to travel to your city, some travel points, a note on foods, culture or festivals. Most of the speakers who travel, love exploring the local places. Do not forget to talk about the hotel and basic facilities too.

Need more details? Setup a call and I can share more directions! :)