Put the benefits in the margin. It allows people to quickly “scan” the letter for benefits, and then they may be encouraged by a particular benefit to read. This technique is now considered “best practice” for many financial services companies.
Then put an 800 number or call-to-action right underneath them. If the benefits are compelling — people may not have to read the letter at all!
Test different paper. Vodafone in New Zealand produced a highly successful package on computer printout paper (to demonstrate it was cutting the cost of your service). We just presented a letter written on a yellow legal pad for the New York State Bar Association.