For instance. I've never really paid attention to white vans, but when the DC sniper was at large and they stated that he's probably shooting from a white van, white vans seemed to appear out of no where and be around every corner!
It's called priming, or attentional priming. It is a well-studied phenomenon of attentional shift.
Interestingly, priming work both ways. That is, a sensory stimulus (like the white van) can in some cases cause you to increase your attention to similar (white van-like) stimuli, or it can cause you to ignore similar stimuli, depending on how the initial stimulus is experienced.
You heightened your detection of white van-like stimuli because you were told that they were a threat. Likewise, you may have covertly suppressed your detection of red sedan-like stimuli containing nubile shapes in order to sharpen your alertness for white van-like stimuli, because the red objects could cause a potentially lethal distraction.
Priming is also used for some kinds of jokes. Jokes with pun punchlines rely on perceptual priming where the listener is led to form a cognitive concept about a word and then has the meaning shifted for humorous effect.
I believe that priming is the culprit for semantic satiety as well.