Use a standard format or template, nothing fancy.
Get to the point, specifically state your objective.
Keep it brief and simple.
Focus on the company you are applying to.
Do not re-hash what is in your resume.
Present complementary information about what you can offer the company.
Do not use humor, stay professional.
Make no mistakes, keep it completely free from errors.
No fancy fonts or colors. Use Arial or Times, 12 point.
Stick to one page. Remember that people are busy.