Make a different cover letter for each company, adding information about the specified organization will show the employer you investigated about the business and that you're really interested in working with them.
Make a list of what skills make you different from the rest, what kind of vintages you can provide to the company if they decide to hire you.
Mention if you had similar experiences to the ones the job you are applying for will provide, where you gained that experience, what tasks you developed and how you would accomplish them.
List your relevant skills, those who represent you better, every employer wants to know more about the achievement of job seekers.
Content is the most important part of the cover letter but so is the layout it has, make it clear, use bullets to describe some points, and try to fit all your data in one page only.
If you waited for more than a week and still don't receive any answer, don't be afraid of contacting the organization and politely ask if your resume and cover letter have been received.
Be specific on what are you looking for, this is the reason why a cover letter is important, it lets you explain if you're looking for a summer job, a part-time job or if you want a permanent position.
Specify how you knew about the vacancy, via newspaper, advertisement, mail or a suggestion from a friend that works in the company, sometimes it's a good idea to mention someone that works there.
The cover letter is read first, so it has to be well-written because after this the employer will decide if he/she will take a look at your resume.
List your personal skills, attitude, enthusiasm, communication skills and motivation.
If some information was specifically requested in the job advertisement and it's not on your resume, you have to mention it through the cover letter.