Put vital information in bold letters. Make everything easy for the reader. Most of all, make it easy for the reader to respond to your offer. Give them credit card, check, money order instructions for ordering. Try to accommodate them in any and every way possible.
People are very afraid of commitment.
So don't make the reader feel 'too' committed by responding to your offer. Give them a toll-free phone number to call or allow email or form ordering for those with web pages. Almost all people have better things to do than read your sales letter. They will only respond when there is something real and tangible in it for them. A poorly written sales letter is a waste of your time and money and will alienate your potential customer.