To write an effective cover letter, you must explain who you are, why you want the position and why you are the best choice for the position. The tone of the cover letter must be formal, yet the letter cannot be generic.
Do not simply restate the information on your resume. Go into more specific detail about positions that are relevant to the one for which you are applying. Because you can use full sentences rather than bullet points like you would on a resume, you can elaborate much more.
Do not make the cover letter only about what company could do for you. Instead, make it clear what you can bring to the company. Be sure to research and have a deep understanding of the position at hand so that you can explain exactly how you could contribute if you are hired. Additionally, focus on your skills rather than your educational background, explaining how your unique skills relate to the position.
If possible, tell a story about your relationship with the company. Perhaps you dreamed of working there since you were a child or you have used their products for years. Keep this section short and to the point. It is something that may make you stand out from other applicants.