i've generally got in touch with the labels owners first (as scott says) tell em a little about myself and what i do and ask if they'd like a link to some mp3s, while offering to send audio cds if they like what they hear. this has worked for me on a few occasions but sometimes you get ignored.

other than that i've gotten releases through knowing the label owners and having a good relationship with them (calver, fatcollective)

good luck whatever way you choose.