1) respect their time, but don't forget your own schedule;
2) ask their details asap: high res picture, bio, visuals;
3) rehearsal, rehearsal, rehearsal (consider using Skype also);
4) know the order of presentations, who is before and after each speaker (so everyone can know what the other speaker will talk about);
5) Inform each speaker their presentation time (it`s always good to know who enters the stage);
6) have "angels" to look after each speaker, or at least the 2 in the sequence to enter stage (give their cel phone # to speakers, they may need them);
7) take care of everything related to their transportation (from their place to the place of the event, from the event to the hotel etc.);
8) visuals: have a designer to adjust visuals;
9) be able to respond asap any requests, specially from those busy speakers;
10) Speakers' assistants are your best friends!
11) Don`t be afraid to adjust their times and even edit their content.
12) Don`t forget to tell them: if they explode time on stage, the chances to go to TED.com website are reduced to zero.
13) have a list in your hand about each speaker, their bios, their phone contacts, their email addresses and assistants. You may need sometime (to speak to a PR, to the press, to some sponsors.
14) enjoy your time with them and try to create mindful questions, the ones which challenge them.
15) read everything you can about your speaker: books, blogs, ways of thinking, press, news.