The basics are to choose maybe 6 labels of different calibers and styles. You should look for opportunities on small labels.
Larger labels watch the smaller labels for new talent.
The larger labels are getting so many demos a week, they don't simply don't have the time to really listen to everything. In general, they are already in contact with the people they are likely to sign in the future. In order to be one of those people, you have to develop yourself as an artist and rise to that situation. Choose labels you spin or that other people spin. Sit down with boxes of records and take notes.