I think the most effective way to let door to door salespeople know I'm not interested is simply to leave the door shut. I don't answer the door if I'm not expecting anyone. I have a glass front door with an opaque curtain over it, and a glass storm door, so if it were, for instance, a police officer at the door (or a Girl Scout or Boy Scout raising funds) I could tell that and answer it, but otherwise, I'm not going to answer the door. Some posters might think that's rude somehow -- especially if my car's in the driveway so it's clear I'm home -- but I do not ever feel obliged to answer the door or the phone for that matter. We get a lot of folks trolling the neighborhoods seeking landscaping or tree trimming work and I would end up wasting both their time and mine if I felt I had to respond to every knock.
I would never let anyone inside the house to demonstrate anything based on a cold call at my door. Not safe and the technique has been used around here to scope out houses for burglary or even as a way to enter for home invasion robberies with the owners right there -- sorry to say it but it's true and we are in a very nice area. In fact this has been in done in some very low-crime, high-income areas close to us. But the criminals know it's a nice area where people will feel they need to be polite and answer the door or engage with a person who claims to have a product to sell.
Now if I could just stop people from leaving business cards, restaurant menus and brochures in my storm door, mailbox, under the mat, stuck in the front door itself (which means they opened my storm door, which bugs me).It feels so invasive. I actually have up a small sign saying not to leave these things anywhere but they get left anyway. One friend had to put up a large "NO SOLICITING" sign on her front door, things got so bad for a while, but she too still gets the paper junk.