While your resume shows what you did, your cover letter can show a prospective employer who you are. It can be a chance to show off your personality and let the people reading it know why they should hire you.
1. Don't send a form letter
It's OK to have a base letter you work off of, but a cover letter should be customized for each job you apply for. Address points from the job ad that make it clear you're putting actual thought into why you're a good fit for this particular job.
2. Avoid cliches
No employer will be impressed by you saying that you're "a hard worker who always goes the extra mile." Avoid generic terms like that and offer examples of real-life things you did that relate to the job.
3. Don't skip questions
If the job posting asks you to explain why you think you're a good fit for the job, answer that question. The same is true if the ad asks you to explain how you might handle a certain situation.
Employers ask because they want to know the answer. Ignoring these questions suggests you may not be great at following directions or carrying out work tasks.
4. Don't be too personal or too silly
You want to show a little bit of personality, but going too personal in a cover letter is a bit like sharing too much on a first date. You might, for example, explain a gap in work history using the term "family health crisis," but avoid the specifics, which might make the potential employer uncomfortable.
You should also avoid being too silly. It's OK to use some humor, but you don't want to turn your cover letter into a comedy routine (and there's a good chance that whatever "funny" angle you take has been tried many times before, so you'll likely get nothing but groans).
Stick to the job
Your cover letter should connect the dots between your resume and the job being advertised. Use it to answer any questions the job ad asks that won't be obvious from your past experience.
If you're lacking something the company asks for, address that and explain how you might overcome your shortcomings. A cover letter is a tool that can land you an interview even if your experience is not quite up to par. Take every one seriously, and use it to show the person making the decision that you deserve their attention.