Be polite. Remember you are contacting real people.
Lots of DJs will love to receive promos but do your research and only contact someone if you think they are likely to be interested in your music.
Write as short an email as possible.
Have your music available to stream and download. I use Bandcamp to share private streaming links, which are great because you can see how many people have visited the link. For downloads, I create a folder on Dropbox containing WAV and MP3 versions of the music, cover artwork, and a press release.
I don’t bother including a press release in the email; you can include it in the Dropbox / Google Drive folder and add the text to your streaming page. I usually just write a one-sentence explanation of what I’m sharing (my new single / album), when it’s coming out, and where to stream it /download it.
Metadata
Make sure all the metadata is included in the files you are offering for download. Metadata should include: Artist, Title, ISRC, Composer, Label etc. (I use a free Mac app called Tag Editor 2, but there are lots of free apps available). I include a press release and JPEG image of the cover art in my Dropbox folder. I include the press release in the private Bandcamp notes.
Don’t pester people!
If I don’t hear back from someone, I don’t chase the emails up. Not getting replies doesn’t necessarily mean they aren’t interested in your music. Some people might be okay with getting multiple emails from you, but lots won’t.
I include a note at the end of my email saying to let me know if they don’t want to receive music promos from me, and I’m very happy to remove them from the list if they ask.
Monitoring
If your music is available to stream on Apple Music, then the Apple Music for Artists service can provide amazing information on any radio plays you’ve received. I find this a very useful feature for keeping track of radio plays.