In everyday language, clichés are simply common expressions that are an easy way to get one's point across. For example, saying, "He really put his foot in his mouth" is a convenient way to make the point that "He said something that he should now regret saying."
What's acceptable in spoken language can be offensively bad in writing. Good writing must be original: you should instead always aim to state your ideas in engaging language and from a fresh perspective.
In addition to the general clichés of the English language, you have to watch out for those that are more specific to the application essay. The challenge here is that these themes have become clichés precisely because they are valuable and significant, so you don't want to ignore them. You simply have to find fresh ways to convey hackneyed ideas. The best advice is to be as specific and personal as possible, thereby emphasizing your uniqueness. The following is a list of some of the most egregious clichés, within the context of a bland statement:
"As I finished the race, I realized I had learned the value of hard work and appreciated the fact that I could accomplish anything if I set my mind to it."
"Working in this atmosphere made me appreciate the value of diversity."
"With each member contributing something valuable to our purpose, I soon recognized the importance of teamwork."
"As the young child embraced me in gratitude, I discovered the true value of making a difference in people's lives."
"That summer in New York truly broadened my horizons."
There's no way to reword the above sentences to make them significantly stronger. The problem lies in the very approach the hypothetical writer of those statements has taken. A reliance on clichés is usually indicative of superficial ideas and telling instead of showing. The only way to improve upon the above sentiments would be to enrich them with concrete details and add depth using a more personal perspective.
"I want to study medicine because it combines my love of science with my desire to help others."
"Seeing how a smile could brighten the patient's day showed me how important such simple gestures are to helping patients."
"As I watched the doctor comfort his patient, I realized that the physician must be able to heal both emotionally and physically."