Here are some tips from Ted Nicholas (he sold > $500,000,000 worth of products (mostly books, tapes, etc.)) and he is one of the best experts.  I advise you to buy his book (Magic Words That Bring You Riches).  Below is my short summary of his materials (in my words mostly).

To be successful you need:
Product
Strategy
Passion
Energy (more water, low fat)
Good mental food
Take action

Don't try to please everyone - narrow your focus
Don't try to "find a need and fill it - instead find a WANT.
 

How to write headlines that make sales soar

Headline - 90% of success/failure of any ad or sales letter
(also teaser copy on the envelope, title of the book, etc.)

Headline:
Attracts attention (like marquee does for a movie theater)
Communicates the strongest benefit
It answers the question: "What is it in this for me?"
Sets the tone for the offer
Selects a right audience

To write great headlines with a marketer's style, first identify benefits and features.

Benefits - answers questions "so what, what it does for me?"
   Use strong action verbs when writing down a benefit

Feature - facts about the product that help build credibility

Best way to capture the benefits is by studying the product - and writing benefits on 3x5 cards in a headline style (answering "so what" question).

The obvious can often be your key to wealth.  Most marketers miss isolating the obvious benefit of their product.

Finding the biggest hidden benefit can be even more powerful technique than the obvious benefit.

You can create headlines that do not come from examining the product

Ask yourself: "If I had unlimited God-like powers and could grant my prospect the biggest benefit I can imagine, what would it be?"
Never change a headline unless you improve results with a new headline.
Test, test, test.  Make A/B tests.  Test 1 parameter at a time.

The use of a photograph with a caption can sometimes improve results.

Headline should not be longer than 17 words.

Favorite words to be used in headlines:

Announcing
Secrets of
New
Now
Amazing
Facts you …
Breakthrough
At last
Advise to
The truth of
Protect
Life
Here (is the way)
Discover (your benefits)
Do you (make these mistakes …)
Bargains (stock market bargains, …)
Yes
Love
Hate  (Don't hate yourself)
How much (is one good … )
How would (you solve this …)
This (could be the …)
Only (way left for …)
Sale ( … )
FREE (Role-Royce)
You (your, …)
How to

Good headline must motivate by stirring an emotion

Use words which are colorful, vital and strong

Use short words that create images or pictures

Eliminate most adjectives and adverbs

Avoid headlines that merely create curiosity.  Prospect will feel deceived, will stop reading and will not order.

Do not try to be catchy or cute.

Other elements of good headline and ad:
  Don't capitalize all letters - use Capital and small letters
  Use quotation marks
  Use large first letter (drop letter)
  Use lots of sub-heads (sub-headlines)
  Use serif-type font
  Put a copyright sign
  Limit use of reverse printing
  Use caption under pictures and photographs
  Use name (your name)
 

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Writing and ideal sales letter:
Clarity
must flow
passion, appeal to emotions
sales come from enthusiasm
believability, specificity
close the sale

before writing:
  be rested
  eat sparingly
  exercise moderately
  do something you enjoy
  cut off the phone
  count your blessings every day
  prepare yourself - get into a peak emotion state

Spend 50-80% of you time on your headline

Prepare order card (your offer) - beautiful, filled with benefits

Use a courier typeface in body copy of the letter (in print)

Remove all boring parts

Read your copy out loud - how it flows

Put your logo at the end of the letter

Good way to start - anecdote (2-3 sentence story or quotation)

Every paragraph should be short.  Use short words.

Ask about each paragraph:
  Is it hot?
  Is it exciting?
  Is it important , but boring
  Is it boring

Offer FREE bonuses (bonuses should be valuable by themselves)

Close the sale

Include a guarantee (the longer - the better)

Include PS (restate and enhance offer and benefits)

Make signature "reflex blue"

Find good copy - and rewrite it by hand to practice

Other elements of good copy:
  don't exaggerate, be believable, admit some flaws
  don't put too many graphics or different type faces
  be respectful
  include as many benefits as you can (don't cut on benefits)
  offer should be clear - don't confuse the prospect
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