This may sound a bit naive to people who have been in the business for long, but when I decided to expand my freelance translation work via proz.com I wasn't all too sure about what belongs in a good application letter (or rather e-mail) and what doesn't. Of course an individual letter responding to the ad is always best, but there certainly are some rules you should stick to. I am not sure that I got them all, but I thought if we worked together here we could give new members a real help.
What I did find out:
Write to the person mentioned in the job ad (if there is a name mentioned), not to "Dear Madam or Sir"; Attach your CV; Include the link to your proz.com Profile, especially if there are WWAs; Mention, in short, your education, but don't expand too much and focus on experience relevant to the assignment you apply to; Name your rate.
Then the question is, should you offer a free test translation or not? There is another thread about this. I think it is a good idea to offer, but after having read the opinions of the others I think a test translation is only in order if it's only 200 words or something like that - and if the company has already agreed to your rate and there is a real chance of you getting the job if the test is ok.