I think everyone here has said everything needed. If you put in enough work and noise they will be contacting you. Network, Producers/Engineers tend to be in the background alot of the times (well some of us) but even so we hold the strings. Go to events, meet other producers, engineers, artists, Djs, promoters etc. Develop a brand and promote yourself thoroughly. If you want to talk business then contact these artists about business. Don't just say "hey I got some dope beats lets make a song." There NOT going to even pay attention but if you come at them like "I've got a hot record I feel you would be great on with the concept (written and referenced). I've got an idea of a music video and believe this could be a hit." Of course you would simplify and word it differently but you get my point.
If you say @rickross "I got fye beats homie. Me & you need to link up for real tho. Str8 flames frm da boy. HMU exclusives going fast..." This is a FAIL.
Now keep in mind this is a give in take thing. so please COPYRIGHT ANYTHING YOU ARE PREPARING TO SEND. If you have a hot beat, wrote some lyrics and the concept and even a script for the idea of the video copyright it all before you send. The artist may not even respond at first, but a few months down the line you may hear/see that video all over the place! its worth a shot. If it does take off then your protected and the label should contact you to get the tracked out beat w/o the tag and stuff. At this point then YOU hold the cards and can negotiate (ent. lawyer)
I hope this helps. If you talk business, then they will respond. Perhaps not the next day but eventually. Get in hold of there management. NOT THE ACTUAL ARTIST. If you need more advice you can follow me @Trilltrax