I have found the best results are achieved when you find someone in a company--just about anyone--who can refer you to the right person. Then you can call and say, "I was referred to you by so-and-so." You may not get to the right person immediately, but that call will generally get you another referral to move closer to the person you want. And having the name of the right person and a referral person really does wonders.
This all takes a little research. Talk with friends, family, associates, or anybody who can give you the name of someone in the company. It doesn't have to be a manager or an executive, just someone on the inside to move you one step closer the decision maker.
Once you identify the person you need to talk to, make an appointment for a face-to-face meeting. That meeting will normally tell you the process for getting business with the firm, or when bidding occurs if they run on contracts. Either way, your foot is in the door and you've got a shot.
Joe