It may seem like a no-brainer but if you have any technical equipment you have to deal with at your wedding (ie sound system, slide projector, computer) test it out BEFORE the wedding is underway. I've had weddings that have ground to a halt whilst everyone frantically tries to get the laptop with the music for a presentation to boot up (note – put any music onto two iPod/MP3 players so you always have a back up). Or a home-made movie that was edited in such a rush the audio didn't work or a slide presentation that was loaded into the projector upside down.
Delegate someone to be your 'techie' (someone who is not in the wedding party as they will have enough going on) and have them coordinate with the presenters testing out all equipment a couple of hours before the wedding and ensuring they bring whatever hook-ups, wires, cables they need.
A few small steps can keep your wedding from falling into the Abyss of Technical Meltdowns.