I have unique skills and experience that will help your company right away. I hope you're as excited about this as I am. The accompanying resume should then prove your case. Put another way, the cover letter is the advertisement for your resume. Focus on the needs of employers and how you would solve their problems.
Employers have problems.
Display knowledge of the company. If you offer well-researched suggestions that will work right away for a company, they will call you. Briefly state your best qualifications and achievements. Don't spend a lot of time rehashing your resume. But do include enough tidbits to generate interest in the mind of the reader.