A friend of mine who is head of a Human Resources (HR) department in a big media company (publishing) always has the best horror stories. It's because they are so bad that they are so good. A few of these mistakes may look like common sense to insiders like she and I but come to think of it they are very acquired skills.It really breaks my heart that with 12 years of education (at the least) and about 18 maximum the education system doesn't teach young people how to job hunt. Here's my insider's two cents on the issue
1. Absolutely no smiley faces. These seem to be the number one thing that annoys hiring managers and serious business people. Not on the email, on your CV and definitely not on your cover letter. This will show professionalism and that you can be trusted to liaise with clients, take minutes (and jump however high you are required to) with grace.
2. Make sure you have the correct contact information. Most companies have jobs@them or info@them email addresses that are put in place make sure all job applications go to the correct place. There is nothing quite as disheartening as getting troves of job email sent to your personal inbox.
I'm sure you don't wan to miss out on a great job because the hiring person was in one of those "If they want the job bad enough they'll send it to the correct address" moods. In some companies it is even policy.
3. Have the correct information. If they give a reference code on the job post advertisement, quote that in your subject line. Make sure you tick all the boxes outlined on the job description. Do some research about the hiring person (this information is often listed on company websites.) Dear Madam has more appeal -- to me -- than "Dear Sir/Madam." but then again I love people who do their research.
4. I always feel like I have to say something about social media. It's easy to assume that we've all learned to keep personal lives (social networks) and our professional lives apart, but most haven't. Make sure your Facebook content is set to be viewed by only your friends. The only time to include your blog or Twitter handle is if it relevant to your field, even then don't include it unless part of your job description requires and active social media life.
5. Ignore me. If you feel you are that good that once the hiring manager sees what you are capable of they'll hire on the spot, then by all means do send her your portfolio every other week. Do put "looking forward to hearing from you xox." at the bottom of your email. The above were just basic principles on professionalism.
Do what you love.