I can't stress enough how important it is to prepare every single detail for the event to be a successful one.
Even the smallest thing can become painful experience not only for your attendees but you as the organizer, too. Even if the content is great.
This morning I've been "lucky" enough to be part of the event, where NOT ONE of the presentations worked right (most of the content was great though, but it was still painful).
Very often it can be not your (organizer's) fault but here's a list of small things (not necessarily in order and definitely not all) that should matter for you as an organizer (and speaker, too):
Prezi: it might help to make your presentation look more "fancy" but does it really help? I'd rather always say no than yes... What you say is what matters, not how you go from one bullet point to another (which, for this matter should be as well forbidden);
Presentation should ALWAYS support the speaker vs. speaker should be dependent on his presentation. That's just wrong. Imagine if you suddenly can't change slide and you become like a fish without the water not knowing what to say next. I always prefer only picture or one-word slides as a hint for the speaker;
If you speak in English, make the slides in English (no English and Lithuanian or any other language mixture);
Please, communicate and then repeat, repeat, repeat and then repeat once again that the presentation resolution should be adjusted to the one of the screen (usually it's 16:9, so please, no 4:3!)
Clicker: don't play with the clicker. Sometimes it seems that it's a TV remote control;
Check the presentation beforehand. Ask your speaker to arrive at least few minutes earlier and have time scheduled to do a tech run through to see if everything is working properly.
And if it's not, you can always make a joke on it and ask the speaker to start all over again (don't apologize for the technical issues--it looks pathetic). You should definitely envision plan b and plan c if such situations happen.
To be continued... if you have more tips to add, feel free to leave them in the comments below!