I feel your pain. I was once hired by a landscaping company to increase their business. I've also done roofing sales, which required door-to-door knocking as well. IMHO beating doors is the hardest sales tactic there is. Dealing with all the rejection is what eventually caused me to decide to go back to school and swear never to do commission sales again......ever. Some people love door-to-door sales and are quite good at it. In the roofing company, I worked alongside salesmen making 6 figures. Unfortunately I got so fed up with people slamming doors in my face that I walked away from it. Money isn't everything and you couldn't pay me enough to do it again.
I completely agree with what llama said. I will add that IMO approaching businesses is much easier and can potentially be more lucrative than residential doors. Start training your eyes to see trees whenever you drive around. Any business with trees on their property has someone that cuts them. Think apartment complexes, industrial places, nursing homes, basically any place that has trees and a door. The reason businesses are easier is that it tends not to be as much of an invasion of privacy than going to someone's house where there is a probability that someone is home in their undies. Also, the gatekeepers at businesses usually have to be polite when they reject you rather than calling you an asshole. The upside is that, if you can either undercut the competition or somehow convince them that your product or service is better, you can have them sign a year contract and grow your company one business at a time.
Residential neighborhoods are a different animal. You have to deal with people getting angry and even threatening for the sake of protecting their privacy. With homes you're going for quantity rather than quality. It's like taking a bucket with a hole in the bottom and trying to keep it full of sand. You'll lose customers frequently for whatever reason, but it's okay because you're still cramming in more.
A good way to get warm leads is to sign your business up for a lead generating service. There are many out there and they do cost money, but you get leads who are seeking YOU out rather than you seeking THEM out. It's all done online.
Good luck with your new business venture. If you have further questions, post em.
Be excited about what you're selling and don't forget your goal.