A poorly written cover letter can do irreversible harm to your resume presentation.  Remember the cover letter creates the important first impression to the reader and why they should evaluate your resume. If the hiring authority is not impressed with your cover letter they may not read your resume at all and you will definitely not get an interview.

Do not beg in your cover letter.  There is a fine line between a dignified request within your cover letter to be considered for a job and begging.  Be respectful in your cover letter but not a wimp.

Do not brag in your cover letter.  Another fine line you need to walk while writing your cover letter is between a considered presentation of your job skills and a pompous pontification.  You want to impress the reader of the cover letter with your professional accomplishments and skills with out sounding like you think you are the best thing since sliced bread.

Do not "bleed" in your cover letter.  The last thing a hiring authority wants to hear is how badly you need this job so you don't get evicted from your apartment. Including your personal financial situation in your cover letter is a definite no no.  So is writing about any type of sob story with in your cover letter. The hiring manager doesn't care about what your needs are when they are trying to fill an open employment position, they care about what their needs are. You should outline in your cover letter how you are going to help them fill their need by hiring you.