The first thing I do is to meet with the organizers and discuss their expectations for the opening ceremony, plenary session(s), concurrent sessions, special session, panels, gala dinners, receptions, or anything else that I will be expected to lead. Once I have a clear idea of how they expect things to go, I can try to help everything run as smoothly as possible from the front of the room. I rely heavily on the people behind the scenes to make sure the tech is working, people are in the right place at the right time, and any changes or surprises are relayed to me so that I can make the appropriate announcements. Surprises are bad and make everyone look unprofessional so communication is key. I'm also asking questions and double checking schedules so that I can share the most up-to-date and accurate information. I also check how to pronounce people's names and verify that they made it to the event before introducing them.
I'm also putting out fires as the come up. These include delayed speakers, schedule changes, technical issues, or an invited speaker using their first language when the official language of the conference is something else. Even checking to see if the restaurant can still take us for lunch if we're ten minutes early is something that comes up when you're the one telling people where to go. Other announcements include when to come back from coffee breaks (Top tip: Tell them to come back five minutes earlier that you really expect them to), where to go for meals, when the bus leaves for a tour, and of course reminding the participants (in vain) to turn the phones to a silent mode.
A good presider will also socialize with the participants and help the upcoming speakers relax. We greet attendees from previous events and make them feel welcomed. We often end up checking and revising the speeches that will be made by members of the organizing committee and visiting government officials. In short, we need to do a lot more than just stand behind a podium and read the names and bios of the next speaker. You need to enjoy being in the spotlight if you do this job and you need to be prepared and detail-oriented. Finally, don't be surprised when people talk to you like they know you. They have been watching you and listening to you all day so they don't see you as a stranger. If this happens, I guess that means you have done a good job. Good luck!