When writing a cover letter: TAILOR IT TO THE JOB AND COMPANY YOU ARE APPLYING TO.
Simple. It should take you an hour or so, and yes, it's not easy, but the general rule is your cover letter gets your résumé looked at (along with showing your communication skills), your résumé lands you an interview (along with making certain you have the required qualifications) and the interview lands you a job.
Some advice on writing cover letters:
http://jobsearch.about.com/library/howto/htcover.htm
http://www.soyouwanna.com/site/syws/coverletter/coverletter.html