Don't be dishonest.
This should go without saying, but people often go too far trying to get us to listen to their music.
There is the blatant lying—saying you just got signed to GOOD Music and have a track coming out with Pusha T next week will eventually get found out, and will ensure future emails are immediately deleted. But almost more annoying is the subtle subject line dishonesty. We see "NEW!! Drake ft. Yung Truthful - THOTS DOWN" and we click, excited by the prospect of a new Drake track, and the fresh new artist that he's putting on.
But no. No, no, no. Of course Yung Truthful isn't on Drake's new song, instead he's remixed a Drake song, rapping over a low quality instrumental he found on YouTube. It may not seem like much, but when you have hundreds of emails to sort through, that little bit of dishonesty is especially annoying. Don't do it.