1. Do your research. Find out if the school has any requirements for portfolio admission. Some schools have very specific requirements for submissions.
2. Draw from life. All schools want to see that you have mastered the difficult skills needed for observational drawing. Be sure to include stuff like landscapes, still life and portraits.
3. Be very selective. Be sure that you are putting your best work forward. Ask for feedback from your instructors.
4. Have your portfolio reviewed by professionals BEFORE submitting. Visit a national portfolio day. You can get more info from www.portfolioday.net
5. Practice talking about your work. Talk about why you created the work, what inspires you. Talk about why you selected the work to best represent you.