It's important to think of a CV and covering letter as one unit. Call it 'a job application' if you like (I nearly putting an apostrophe here but thankfully re-considered). The two need to work together. To be precise, one should lead to the other. The covering letter should point towards the CV.
Advertisers talk about the importance of a 'call to action'. The call to action in a covering letter is to 'read my CV'. It's easy to think about how to construct the perfect covering letter by breaking it down like this:
1. Confirm the role you're applying for.
2. Provide any information that enhances your application (that isn't in your CV).
3. Convince the recruiter to read your CV (the call to action).
It really is that simple. Don't get bogged down writing reams of information in the covering letter. Keep it succinct and snappy. Don't repeat information. Have a clear call to action.
Follow these guidelines and you can be confident any recruiter will be pouring over your CV and giving your application serious consideration.