Your medical school personal statement needs to sell. Every single word has to sell your reader and answer the question: "Is it worth my time to read one more word of this essay?" To be persuasive is to sell your reader on your next sentence, your next paragraph, your next idea of your medical school essay.
Now you are looking at the remaining 20% of your word limit being your conclusion. Percentages may vary if larger word limits are being offered. Ensure you state why you are interested in this field of study; state the key points from the body of your essay (e.g. accomplishments, experience and interest). Ensure that you keep it brief, to the point and leave out the clichés. Just as you started your personal statement with an attention grabber, make sure you finish your personal statement with one or two sentences that are positive and leave the reader remembering your statement while they are reading the next few.