What are some good ways to get my foot in the door if I want to shoot for a well-known publication?

same here, its like a dream of mine working for national geographic, im 20 but had a few photography cles, i talk to pro photographers at there studios and sometimes they dont even hire people who had one cl in photography, its based on your own knowledge. Build up a portfolio. Get to the source, one time i email Nat Geo asking what would it take to work as a photographer, they never emailed me back lol. It is possible to make a living if you stick to it! 2 years of shooting, you probably still have a few things to learn. Ive been doing it since i was 17 and im 20 now and still dont know that much, yet i read books everyday and once in awhile shadow a pro photographer.