Whatever you do, don't wait until your inhibitions are down before sending the cover letter and resume. If you can't help but hem and haw over the cover letter, you're not going to submit anything until late in the day. Then it will be near bedtime, and you'll be more likely to put stupid obnoxious-sounding things in the cover letter. If you have this problem, it might help to make a game out of writing a simple, mediocre, polite cover letter, instead of trying to make a good cover letter. My response rate went from 0% to 100% when I did this. If you're not a naturally obnoxious person, maybe you won't have this problem.