Technique: Shooting Backwards for Forwards
Any easy effect that just about anyone can do is to take footage and reverse it in post. This can be used in one of two ways: to make something look like it's moving backwards (duh), or to take something shot backwards and make it look like it was supposed to be forwards.
As a simple effects shot, this can yield some cool results. In my short film, Middle of Nowhere, I wanted a car's digital clock to look like it was ticking backwards. I simply shot the clock normally, then reversed the footage later. What I really liked was the colon between the numbers was blinking as if it was running forward, then GASP! it went back in time!
This can also be a way to safely pull off shots that are inherently dangerous. Remember in Predator (1987), when the alien pins Arnold's neck to the log with the wrist-mounted knives? Close inspection will reveal that the knives started in the log, and were then pulled out, insuring Schwarzenegger wouldn't be injured.
Sometimes this effect is played out as a gimmick for the entire piece as in the above clip from Top Secret! (1984) or the below music video from the band Mute Math. What's impressive about these is the obvious amount of rehearsal that must have went into them. The lead singer even practiced how to say the lyrics backward while shooting, which end up matching pretty well when reversed.
Another mind bending turn is when normal events are combined with a backwards-moving actor, which ends up looking like he's moving normally and the rest of the world is moving in the opposite direction. This was the effect in Leon Prochnik's short film The Existentialist (1963), which goes on too long, but creates a bizarre world in seemingly normal surroundings.
Just remember, when you attempt things like this that you'll have to create an entirely custom soundtrack. Since you'll be reversing everything, your audio will be essentially useless (unless your actors are supposed to be speaking Swedish). Of course, if you're making a music video, this is a moot point.