I've been doing Newsletters for about 8 years, they get changed everytime but what I do find is that if you don't get the message across the client will scan it and throw it if they can't find the info they want with in a set time.
Headlines and sub Headlines
Are the way to combat this, look at a newspaper whether it be the Sun or the Times they know how to keep you reading even when the content can be dull. Big long paragraphs may tell the client what YOU WANT them to hear but it will be lost if they don't get that far.
DJ's do it all of the time in your ear
How infuriating when a DJ on your local radio station says " Can you remember this big hit of 1988 coming up now...........after the break"
Keeps you listening while they play the adverts a bit like a P.S. at the end of a letter. How many of you scan the letter and only really read the P.S.?
One thing I DO disagree with is 'the longer the letter the more the response' I have tested this on 3 occassions, 1 with a Joe Polish letter and 2 of my own and they have all bombed, you have to keep it punchy to some extend.
Hope this helps and carries on from what ian is saying.
Shaun