Being a music producer as well as an editor of a blog, I have the best of both worlds when it comes to music submissions. Many sites have their own guidelines for submissions and these should always be addressed but on the chance that they don't, here are a few tips you may want to consider in hopes of getting your music on your favorite online publications.
1. Keep the subject simple
Capital letters are a no-no. Even if the reader would accept it, their email's spam filter may not. Keep in mind what you say and how you say it. Something as simple as "Music Submission: Your name - Album/EP name" will still get their attention and most likely stay clear from the clutches of the dreaded spam filter.
2. Don't describe yourself by the compliments of others
This may be obvious, but it's worth repeating. In the words of Reverend X, "your thoughts ain't my thoughts". Whatever someone said about your music doesn't necessarily ring true with another listener. It's best to refrain from comparisons to major artists or your favorites for the sake of sounding generic and like every other rapper.
3. Attach a photo and relevant links to your work
Unless they specifically tell you not to, give them as much information about yourself as you can without writing something you'd expect to read for your obituary. Sending a photo means they don't have to source one, which saves time and that means more time listening to your music. Giving them links to your work and where you are on the internet is also handy should they want to keep in touch or make a note of your future releases. Giving them Facebook and Twitter links also means they can link you in tweets that go out if they publish you, increasing your exposure.
4. If they haven't written something for you, don't harass them
This goes against what people tell you in relation to applying for jobs, but the difference is bloggers don't actively advertise for artists to apply for space. As many bloggers will attest to, we have an everlasting backlog of features and a lot get deleted before a second of audio is heard. There could be a chance yours is amongst the deleted but if not, be patient.
5. Make sure your music is stream-ready if possible
Having spoken with bloggers, a common gripe is the lack of previewing music before deciding to download. Having releases purely on Hulkshare or Mediafire just won't cut it, especially if you haven't released anything before. Setting up a Soundcloud, Audiomack or Bandcamp account takes very little time and comes with plenty of opportunities to gain a fan base or add to your current one.