OK, first off the first impression is everything. Make sure your tracks are 100% produced and mastered. If you send 10 tracks make sure at least 4 are FIRE. So perfect your craft. This is no joke. (BUT MAKE SURE HOW MANY TRACKS THE LABEL WANTS SENT) You gotta stand out from the rest. Record companies get 100s of submissions...
Second, it's really hard to get a production deal with the majors. The industry is full of producers right now. That's why you gotta have your stuff polished to the 100th power!
Third, big record labels don't take unsolicited material. Some of your work might get by them. But most likely it won't. That's why you need a manager to do that work. When you get a lead, get your manager to contact that person. Have your manager introduce the company and get the down-low on everything. That way, when you send your tracks off, the label already has it's eye out for that package. By doing that, you already have giving yourself more of a chance. DON'T GO THROUGH THIRD PARTIES!
Make a good list of things you have done producing wise. And I mean good stuff. If you DJ at big clubs put that. If you submitted tracks to other labels name them. Even if tracks didn't get picked up, this shows you are on the move. If you got tracks on the radio (even behind commercials) name the stations. If you volunteered at youth centers for music, add that. By adding everything in your out-line, it shows you are serious.