Jay:

One thing that a lot of label owners always seem to like is if you know a bit about what they do and you personalise anything you send.

e.g. send a covering letter explaining why you and your music fit in with their label.

It's just like going for a job... if you can show you know a bit about the company - i.e. done your homework - and put a good case forward why you'd be an asset, then it looks good.