Write to, and only to, the essay topic. Know exactly what you are being asked to write about. Some personal essays are more open-ended than others. Some ask you to address a specific topic about yourself and your beliefs.
Follow the formatting instructions of the essay. If it only asks for 500 words, stifle the urge to go on about how great you are for 1,000 words.
Use humor sparingly. Write seriously and professionally, but throw in a joke here and there to lighten the tone.
Leave out the woe-is-me component of your story. State the facts of any hardships you have overcome, but focus on how you surmounted those obstacles, how you triumphed and came out on top. Complaining and whining about how unfair your life has been shows a poor attitude in general that the admissions board may find unattractive in a potential future student body member.
Focus on creating a confident, not arrogant tone in your personal essay. You can be sure of your abilities, but there is no need to list every honor you received from kindergarten all the way through high school or to use every other vocabulary word you learned from your Advanced Placement English classes. That information is on your application. Admissions board members are looking for a window into your personality and your perspective on life that they cannot get from reading a paper about how you think you are superior to other high school seniors applying for admission.
Write your personal essay in a structured, coherent way. The flow of the essay should make sense, be logical, and revolve around one or two themes you state in your essay's introduction. Use supporting anecdotes, statistics, or other information to bolster any claims you make. Show your ability to analyze critically and to present your arguments effectively.
Relate your personal aspirations to the specific school or department to which you are seeking admission. Avoid saying how great the school is. Try another angle other than, "I have always wanted to be a. . ." Tell the essay readers why you would fit in well with the department or school, and why you would be an asset to them.
Explain any shortcomings in your record, such as poor grades. Tell the truth in all that you say. Avoid glossing over any trouble spots in your academic career, but do not spend too much time on them either.
Proofread the essay at least twice. Ask a trusted friend, teacher, counselor or parent to read it and proofread it. Look for more than just typos and spelling problems. Sentence and paragraph structure, overall flow and grammar are also vital.