If they don't advertise online, then this is probably the best thing to do. Another great thing is to try and get to know someone in the company - i've found in the past that people love it if you know someone who has worker or is working there.
I would say that the most important thing in a general email is telling them that you are aspiring to go into this field - if you're enthusiastic, this will look good. Also, as mentioned by si, calling can be effective - as emailing can be ignored, which can be really frustrating if you do not...