Keep the following "dos" in mind as you prepare your cover letter:
Do customize your cover letter to each job
Do be concise
Do be consistent with sentence structure (person, transition words, etc.)
Do include a valid professional email address and phone number
Do make sure your cover letter looks good and is printed on the same paper as your resume, preferably a good quality resume paper
Do address your cover letter to a specific person if possible
Do refer to your resume
Do state your interest to interview for the position
By avoiding these "don'ts," you'll strengthen the quality of your cover letter:
Don't lie, exaggerate or include something that you would not feel comfortable discussing in an interview
Don't rely on spell check alone! Have someone read over your cover letter for typos
Don't use the word "I" or other first or third person pronouns
Don't use speckled paper that can be difficult to read once photocopied
Don't use curly fonts that may be difficult to read
Don't use large fonts or extra spaces to increase the length of your cover letter
Don't make your cover letter too long