...do you have any suggestions/tips/examples for great covering letters? I know the basics; they should be short, to the point and explain what you can do, but your resume examples were really helpful and I thought maybe you (or anyone out there!) could help here again? I think it's always useful to get other people's views and opinions!
actually cover letters vary depending on the reason for the head shot. I use a typical business letter format. Nothing special. However if I'm sending my stuff in for an audition appointment, then I keep it very short. I mention where I saw the audition posted and what I'm auditioning for. For example if I were auditioning for John in Miss Saigon, I'd say so making sure that the title of the role and show are both in caps and bold. It may be grammatically incorrect but it's better for a casting director who's sifting through hundreds of cover letters to be able to glance at it quickly and say "John, Miss Saigon...this pile.". Casting directors are human and they get annoyed with long drawn out hard to find out what they mean cover letters.
A good portal to look for samples and cover letter tips: The Playwrights Forum > The Art & Craft of Writing > Synopses and Cover Letters. Also: Mary Sue Price published a short guide on the effective communication with theaters: cover letters, letters of inquiry and follow-up emails. It's an article on the Playwrights’ Center portal.