the ground for new artists getting worse more and more, too much, really good ones, of them around, most lables already closed their doors for new artists focusing on the own roster-family ... so demo sending will get frustrating easily, sometimes its seems easier to found an own label instead :-)
but some basic rules generally sending stuff to labels...
- first check any demoguidelines a label give on its website , stick to it!
- check if your sound really fits to the labels sound
- as Inertia said, generally do not send any public stuff, labels want to be exclusive
- send 3-4 tracks, instead of a single one, labels want to listen a demo that might be a complete EP release instead of supporting then with expensive remixers on a single track
- write single mails to single labels, mass mailing will be deleted instantly, I promise!
- put some info in your mail, makes it more personal than just a single link
- decide on your own to send only to one label , wait for response , then send to the next one ... causes much time, but no problem later on to decide which label you prefer if more then one gets back to you
might be a little offtopic, but hope it will help a little on your way...
wish you all the best, keep up making great sounds!