(An example I'd picked up a few years ago now mind, I've never used it fully to know if works)
Dear <sir, madam, name etc>,
Re: Application for <position> at <somewhere>
Please find attached a copy of my CV/application form in application for <the job title> at <company name>.
I have worked in <an appropriate field> for <x> years and have gathered a large range of experience in <this field> which is put to good use as a <whatever you do now or most recently did>. This involved <whatever the job involved> and <some other skills that are listed in the person spec>
In my previous/current employment at <somewhere>, I was responsible for <this, that and the other>, ensuring <this that and the other> was completed to a high standard/in good time to ensure other work flow could be done. This is a skill I can easily transfer to this position as a <job title> with <company>
<insert another two or three skills or requirements from the advert and write something here about how you have those skills, how good you are at this stuff and back it up with actual examples. Use the job description and person spec and show that you've actually read them!>
Thank you for considering my application and I look forward to hearing from you soon. I can be contacted on 07xxx xxxxx if you would like to discuss my application further.
Yours faithfully/sincerely,
Xxxxx xxxxxx