Consult with the guidance counsellor about submitting transcripts and other official school forms.
Consider why a question on the application is being asked, to make sure that you answer it appropriately.
Try to use different examples for each question, using experiences from academic work, extra-curricular activities and work experience.
Show that you've researched the subject/course by demonstrating that you have some of the skills required.
Don't expect admissions committees to assume anything. If you want them to know your unique selling points, you need to tell them!
Be concise and stick to the word count.
Check, and double check, spelling, punctuation and grammar.