What do I need to ensure my presentation will go smoothly?
From less esoteric to more (I've seen all these things happen):


Make sure you know how to "send the image to a projector". Not all laptops do this automatically, and I'm surprised at how many people don't know how to do it. I've also seen people be confused by the mirroring feature on Macs.
Verify that there's a power outlet or some place to plug in your laptop. Driving a projector eats power, and you don't want to drain your battery during the presentation
Turn off screen savers. at the very least it's annoying, and sometimes the screen saver does wonky things to the projector display.
Turn off your notifications (mail, facebook, twitter, skype...). It's amusing for an audience (but not for you) if during a job talk they all get to see a subject header of the form "Interview at University X" where X is not where you're at :)
Make sure you have the right display converters. Usually something that converts your laptop to a VGA is standard. Macs are particularly difficult in this regard. 
Watch out for resolution issues. Most laptops are smart enough to drop resolution to deal with a projector, but sometimes they're not. 
Keep a backup copy of the slides in a portable format (PDF or PPT) on a usb stick. In the worst-case you can always borrow a laptop (from the previous speaker even) and load up the slides. Dropbox/a web page is ok but not great because it requires an internet connection. 


And above all, as David M. R. says, check the setup beforehand if you can. Even that doesn't guarantee a smooth presentation, but it eliminates a lot of the potential problems listed above.