A cover letter is that all-important piece of information that you want your future employer to read, this has to be the first thing that they read so therefore it is your PITCH.
What are you pitching? Yourself, ofcourse and probably the reason as to why you want to work there, and how you can make their company better.
Now, the cover letter does vary from industry to industry, and certain jobs will have their own format of what is required in the cover letter. In public relations, there needs to be a recognition of your achievements – such as campaigns, past work experiences that you have gone well or achieved great results for and you may also want to put down that you are enthusiastic about the job you are going for (note to self – must do research on company beforehand)…
I have found some tres super websites that are ideal for helping you write your cover letter – of course there are…dare I say it…BOOKS…that you can use as well….hehehe…
2. Job Search – http://jobsearch.about.com/od/coverletters/Cover_Letters.htm
3. AC People – http://www.acpeople.com.au/cover-letters/
4. Career FAQ’s – http://www.careerfaqs.com.au/resumes-cover-letters
5. Youth Central – http://www.youthcentral.vic.gov.au/Jobs+%26+Careers/Applying+for+jobs/Sample+resumes+%26+cover+letters/
6. Seek.com.au – http://www.seek.com.au/jobs-resources/get-your-dream-job/coverletter
But ideally a PUBLIC RELATIONS cover letter should look something like this:
I hope this helps you out whilst you are On The Job Hunt!
Emilia 
Emilia, you have mentioned some very useful points in this post and some great sources to find more information, this post is very useful for people on the job hunt. Thanks for sharing!