Your potential sponsor will be able to relate to you on a personal level and, quite possibly, could recognize you from a prior event or conversation. The next step is to figure out which companies you would like to represent and be represented by. Being somewhat new to the industry, I would have to say that my computer was the most helpful in this area. I spent approximately ten hours searching the Internet for motocross industry companies. I look at companies that supplied everything from spark plugs to race gear.
Some sites also list the information on who to contact.
My belief is that mail in the hand is better than a floating email. You never know if they actually received your email or even took the time to print your resume out. Try and mail out all of your resumes at the same time, this way you know when to follow-up. I believe a follow-up call to the Sponsorship Manager is a definite plus, you can always ask to leave a message, this way you are never calling at the wrong time. Following up is an important step to show that you are serious and that you take your sponsorship seriously.