1st purpose - this is most likely ur first impression on them. sell urself but dont over do it.
2 - try to keep it in one full page.
3 - leave out unnecessary information thats just fluff. i.e. - "hobbies: working on cars" or something like that.
4 - quadruple check it to make sure you have ZERO spelling/grammar mistakes
5 - try to make ur past experiences geared towards the industry/field you're going into (education)

this is all pretty much basic common sense. the problem is a lot of people just dont follow it.

p.s. - ALWAYS send a follow up email/call after each interview.