Do your research.
Make sure you know the site you're submitting music to. Sending a rap blog your new tech-house track is never a good move, and suggests you haven't even taken the time to read the site you're contacting to make sure it's a good fit. This research is especially important if you decide to include the much overused phrase, "I think this will be perfect for your site!"
It's simple, but if you put in a little effort and mention something specific about the blog that you're submitting to (like a recent song posted that you like, an article you enjoyed, or why you think your music would fit), it will make the pitch more personal and make you look like you're taking this seriously.