Unfortunately you have to come to terms with the fact that the vast majority of what you put out will go in the bin, no matter how great they look. It's the nature of the beast. They say that a person has to see a mailshot at least 4 times before they even think about responding to it.
That said, it is part of the job and it has to be done. You have to keep at it and make sure you get the point across. It needs to be to the point, not too clever if its the first one you've done. You need to get the message across quickly and efficiently because if it's the first mailshot they've seen from you it wont be in their hands long, so you need them to get your contact details as quick as possible.
Then when you've posted to a certain area, hit it again, and again, and again,...