The most important aspect is to get to the point. The first couple of lines of your email should say what the mail is and what's contained within it. Bloggers and DJs can get dozens of promo mails a day; more than they could realistically listen to, and a huge proportion are completely irrelevent - indie-pop bands sending promos to dubstep blogs and so on. So straight away, your mail should set out exactly what's contained; what the music sounds like, any DJ's that have been playing it out, and so on. Leave the bio and other information until further down.
You should also personalise your email as much as possible. No-one likes to see a blatantly generic mail, and especially if you're looking to get signed or get a DJ to play your tracks, you should explain why they might be interested in you. This brings us on to that classic promo error - the BCC fail. Your author receives at least one mail a day which is CC'd to about 300 other random addresses, from magazine accounting departments to PR companies, and the first reaction is to consider it a low priority - with all those addresses, if one recipient doesn't deal with it, someone else might pick it up. It also annoys people to see that their email address is being sent so far and wide - so make sure you at least use the BCC function!
Fortunately, in the internet age, it's no longer really necessary to have a huge and impressive looking introductory mail. A big HTML email may look clever, if it displays properly, but in these days of instant information, people just want the facts. A jazzy press pic, a lengthy biography explaining how you got your first set of decks aged just four, the reasons behind why you made this or that track - all basically irrelevant. You can cover all that with a link to your website (where, of course, all this information should be easily accessible). It's highly frustrating to be scrolling through an email full of information looking for the one piece that really counts - where you can hear the track!
So what an email should look like? Well, it should be short, concise, and well-written - you don't want people thinking you're as slapdash with your music as you are with your spelling. It should include a brief description of what the music is like, and any recommendations that you can think of - DJs who are already supporting it, labels that have previously released your work, etc. It should include links to stream and download the music, perhaps a recent mix, and links to your website or facebook page where people can find more information if they choose. If you can include any personal gambit to the recipient, to make sure they realise you're mailing them directly instead of just spamming a list you found on the internet, then so much the better. And last but not least, be polite!
