1. Be Unselfish
The tone of your message should not be how you expect to personally benefit (gain experience, learn and grow) from the job -- but rather what you can do for them. What do you bring to the table? How ready you are to hit the ground running? Employers really don't want to hear how they can help you -- they want to hear how hiring you will help them.
2. Be humble
Yes, you are your own advocate, but know the time and place for self-promotion. Reaching out to someone you don't know is a good time to tone it down. Emphasize your interest in the company, and the opportunity you see to specifically do great work with them. Include a few facts to get them interested.
3. Be polite
Mention that you sent materials and wanted to make sure they received it.
Keep in mind that people are very busy and multi-tasking, never give the impression you are annoyed because you haven't received an immediate response to your email.
4. Be pleasant
In person, would you launch into an elevator pitch before saying hello? Greetings are very important, especially when asking for something (which is really what you're doing when introducing yourself regarding a job).
5. Be professional
This is less about "sounding" professional (ie. using jargon, formal letter structure), than it is about respecting corporate culture by:
- Keeping your email concise and as short as possible, and
- Using proper grammar, capitalization, and spelling.