Hi John and welcome to the forum.
You will find all manner of great advise from members who have been thru every stage of the visa process. Including myself. (My fiance arrived here May 2006 - we get married next Saturday - and apply for marriage visa Sept 31st).
You did not say why you were refused in the past, so I cant advise where you went wrong until you give us something to go on. After all you dont want to be making the same mistake twice.
As for your covering letter, we were advised that mine was one of the strongest most informative they had read, and on that basis plus all the other evidence we submitted my fiance didnt need an interview !
All I did was take the advice boklet issued by the Home Office - the one that advises what documentation they require. About 2 pages into this booklet is a list of items that should be addressed in the covering letter...
I took each seperate issue and explained fully what they wanted to hear (know). I also referred in the letter to items of evidence within our file to prove the various points they needed to know. Mail me your address and I will happily send you a copy of my letter - with all the personal blanks in place of course for your own personal details.
Good luck with your second application.