Bright colors, rapid or unusual movement, interesting sounds and pungent smells all grab our attention. These external factors form the building blocks for perception because they send signals to our brains to pay attention. For example, when everything is a similar color the tendency is to focusing on details and either relax or get bored. When a single spot of bright, contrasting color is added you will immediate see it, identify it and remember it. That spot of color will change your perception of a boring or relaxing place to one related to the color. That perception might be "this place is beautiful" if you noticed a flower, or "dangerous" if you noticed a fire, but in either case, the bright spot dominates your perception. The same can be said of other sensory information. Consider unpleasant music or a bad smell in an otherwise beautiful restaurant, for example. (reference 3 exercise 1 notes)
There is a reason that marketers strive to keep their products in front of you as much as possible. Repetition not only eventually causes us to pay attention, but it gives products an aura of authority because familiarity lends authority to their claims. When you stop remembering where you heard that "X product" cleans the best and associate it with excellent results, the marketing campaign has effectively shaped your perception. You see this in politics and teaching as well as in sales, as your perceptions are shaped by repetitive messages about issues or topics.
Like a sudden noise in a peaceful setting, unexpected messages or behavior can dramatically influence our perceptions. Suddenly mean behavior from a trusted friend, for example, might cause you to immediately question your perceptions of that person as might nice behavior from a person you had superficially judged to be unpleasant. Similarly, unexpected claims about products or historical facts can get your attention and cause you to examine an issue, idea or product more closely.
The behavior of others and cultural expectations shape our perceptions on a daily basis. Think about how difficult it can be to begin a class with a teacher whose reputation is one of being unfair or overly demanding of students or a subject that is notoriously difficult. Your perception of that teacher or class and may prevent you from being open to what the teacher actually says or does. Cultural expectations similarly shape our perceptions as they help us focus on what is important in our culture. This sometimes cause tension between people of different cultures in social and business relationships based on a mismatch between intentions and perceptions. For example, averted eyes is considered respectful in many cultures, but it's often interpreted as inattention in America.