Know your enemy.
Making words lose their meaning when you repeat them too many times.
"Semantic satiation" is the name for when you repeat a word so many times it sounds like nonsense. The phenomenon was being talked about scientifically as early as 1919, when a paper was published that tried to determine the number of times a monosyllabic word had to be repeated for it to lose its meaning.
Making it harder to spot your own typos.
When reading, your brain doesn't focus on every letter but instead takes in whole words. That's why you can usually read words even when only the first and last letters are in the right places, lkie tihs. It makes reading much quicker, but a side effect is that spotting typos in your own work becomes much harder, because you're so familiar with it.
One way to get around this is to make it less familiar – try changing the font or printing it out, for example.
Helping you remember things that didn't actually happen.
"False memories" are surprisingly easy to plant. In a simple lab test, psychologists gave people a list of words related to sleep (bed, rest, awake, etc) but didn't actually include the word "sleep" itself. Later, when asked to recall the list, participants mistakenly remembered the word "sleep" as often as they did words that were actually on the list.
This phenomenon is why its easy for witnesses of a crime to remember things that didn't happen. But authors of the sleep-words study see false memories as part of what makes humans clever: "The fact that such inferences can lead one astray ... is a small price to pay for the inventiveness of the human mind."
Making you believe horoscopes are specifically about you.
There's something called the Forer effect, which is named after the psychologist who discovered it.
Bertram Forer told his class he was giving everyone a statement about themselves to read. The class marked the statements for accuracy, giving them 4.26 out of 5 on average. Then – shock horror – he revealed afterwards that all of the statements were the same.
They said things like, "You have a great need for other people to like and admire you," and, "While you have some personality weaknesses, you are generally able to compensate for them," which, when you think about them, could apply to most people while still managing to sound specific. These statements are basically how horoscopes manage to sound scarily accurate for lots of people.