Always be Consistent
One of the biggest mess ups when training a dog, is not having consistency. This could be not using the same command or correcting the dog only sometimes when he does something wrong. This only causes confusion for him and he will not know what you are wanting him to do.
Ex. When training your dog to stay off of the furniture, make sure every family member in your household are using the same command. If one person is saying down, while the other is saying off, he will not know which command to listen to. Or if someone is consistent in not letting your dog on the furniture while another person is, he will not know what to do. Always stay in tune with everyone in the house while training your dog and always use the same word for the command.