Ring, ring, ring, ring, ring and then ring around some more.
Find out who you have to write to. When you write make reference to the person that you spoke to and make sure you don't waffle.
Try to grab their interest with your first paragraph and once in a blue moon you might just hit the target.
Mention in your letter that you will follow up with a courtesy call.
Call when you say you will.
This way you are likely to at least be invited to tender for future contracts (this will be more than just once in a blue moon!).
Trust me :-)