Contacting companies - what's the best way?

[Eireann]

I've just been looking up companies operating in the field that interests me the most. I've sent emails to some of them. I'm offering to work for or with them (I have a business license).

What is the right way to grab the reader's attention with the heading and the message? I want to write something that will be noticed and remembered. And answered. I want to send emails that, at the very least, lead to an interview.