A resume without a cover letter is like a beach with no sea. The two come together in synergy to create a more impressive, rounded application. While your resume shows you're capabilities for the job, a well written cover letter shows a recruiter your personality. It also shows a hiring manager how you explain yourself in writing and gives an insight into how well you'll fit in with the existing team in terms of company culture and team dynamics.
Whether you're applying for an advertised job or you're sending a prospecting application a cover letter will always improve your application if it correctly, a) Introduces who you are and mentions the job you're interested in and b) Emphasises the skills mentioned in your resume, backing them up with concrete, quantifiable examples.
Finish off with a call to action, for example adding a phrase like “I am available for interview at your convenience”.
So when writing your cover letter, make sure you do, and don't do, the following:
Personally address your letter.
To who it may concern isn't good enough. You need names, and while the researching is time consuming, it's definitely worth it and makes a good impression on hiring managers. If you can't find a name on the job ad or on the company website, call the company up and ask for the name of the person you should send your application to.
Research the job and the company.
Don your dective hat and spectacles and go Sherlock holmes on this. Get as much information about the company and the job as you can. Use channels such as investor relations web-pages on the company profile but also 3rd party companies like trade associations, think tanks and market leaders that produce insight reports.
Show your personality
The employer is already interested in your application if they're reading your cover letter, so here they're looking to find out a bit more about you and your personality. Therefore you need to make sure your personality shines through and that you don't sound like any other professional using the same buzz words and application clichés.
Copy your resume
Maybe people get lost in their applications and lose sight of the distinction between cover letter and resume. Your cover letter should not list your past jobs but instead should emphasise your skills to the employer in a dialog. Give examples for the skills you've mentioned in your resume but don't repeat information you've already given them with your CV.
Send your letter without a double check.
Make sure you're not sending your cover letter off before you've properly checked your spelling and grammar. While spellchecker tools are useful, it's important to be aware of the fact that they don't catch all your mistakes. Moreover, make sure you have your information correct. Don't misspell the recruiter's name, or the company's for that matter.
Using the pronoun "I"
You don't need it. Your cover letter is about you and so it's obvious anything you mention refers back to you. Avoid sentences like “I believe I am ..” or “I have excellent customer service skills…”. Using I is bad writing style, instead opt for impersonal phrases like “ Your cover letter is about you, but try to make sure that you don't fill it with things like "Working at Company X enabled me to …”