Send it. That is the first thing. Most of what follows is people talking themselves out of sending, and a tape I never hear is worse than a bad one.
What helps. Two or three songs, not eleven. The one you would open a set with, first. A line about who played what, because I will ask and it saves a letter. Rough is fine. Rough is often better, because a demo that has been polished for eight months is a demo whose band has stopped being able to hear it.
What does not help. An explanation of the concept. If the record needs the letter, the letter is doing work the record should be doing, and I am not going to be standing next to the listener to read it out.
I will be rude about it and then I will play it again. That is not a threat, it is the whole job.