In no particular order...
1) It should go without saying, but don't be rude or demanding. No one has to review your album or plug your project.
Writing "post this video up and give some commentary" generally won't get you what you want.
2) Don't send an email dissing the blogger's current posts and stating that "enough about X, it's time to talk about [insert your project name.]"
2a) This is especially egregious if the blogger has recently mentioned your project.
3) If you send an unsolicited email, be prepared to have your email quoted. You have no expectation of privacy. Especially if the email link on the blog (as on this one) has a disclaimer stating "I will assume your permission to publish your comments unless you indicate otherwise."
4) If you have any requests, especially if upon reflection you prefer that an email you sent which was posted be removed, it's OK to ask politely. Once. Sending multiple emails is rude in general. Bloggers do other things besides sit at their computers waiting for your email so that they can answer it within 10 seconds.
5) Do not comment spam. And, don't have your fans do it either.
6) Respond to questions. If you send an email plugging your project, be prepared for the possibility of a response. Not following up on your email, or worse, ignoring interested questions is foolish and counterproductive. It leaves a negative impression of you and by extension your project.
7) Include your name in your email. Unsigned PR/review requests are pointless.
8) Don't send your entire album, or even a few mp3's if they are unsolicited. Filling up a blogger's mailbox is inconsiderate. Sending an email with a download link is fine.
9) If you're trying to do promotion for a friend, the same rules apply. Here's a bit of an email we received a while back:
Hi, i just wanted to apologize for my last email about my friend [redacted's]We'd ignored the initial email which was kind of rude, full of misspellings, and unsigned. The displeasure with the email was registered by someone else, presumably the artist. Spare your friend and yourself the embarrassment.
album. It was pointed out to me as to how unprofessional and impolite my first email was. Aside from the mispelled subject line and the typical cliches used, the email was indirect and impolite. Im new at this, so please excuse the last email.
10) Be honest. Don't pretend you don't have a connection to the project you're plugging when you do have one.
11) Use spellcheck, punctuation, capitalization and break your email into paragraphs as appropriate.
I'm sure the other bloggers have tips they'd suggest too...