Eat before your dog so "he knows who is boss." This probably got started because trainers wanted their clients to just think about training on a daily basis. One reason that I also hear frequently is to mimic the wolf behavior of the alpha wolf eating first. This will supposedly teach your dog that he needs to behave because you are eating first.
Don't worry about being the alpha, just train your dog! An easy way to think about training every day is to tie it to something people do every day - eat. While a person is eating, they might ask their dog to lie down and wait, which just helps with daily training, but is not a requirement for a dog to be well behaved.
- Walk through doors before your dog -- same reason, so he "knows who is boss." Once again, just a way to remind a person to work on teaching their dog to be patient and to work on control. If a dog walks into a room before you, he just wants to go into the room faster than you, it does not make him "dominant."
- Make sure your dog walks by your side, if he walks ahead of you he is being "dominant." Ridiculous. If you want your dog to walk next to you, train him and motivate him to do so. Both things take a little time and ability. If your dog is not trained, don't blame your dog, work harder on teaching him or find a good positive reinforcement trainer that can show you the proper techniques.
So much of successful dog training is based on working through challenges and figuring out how to motivate a dog to do something. It is really easy to blame the dog instead of becoming a better trainer.