|Murphy is not only Sitting, he is Waiting too.|
When training my dog, I used a word command plus I added a hand signal. For example, I would say Sit and raise my hand out in front almost as if I was saying Stop. As we were training, I had my son push on his behind to get him to sit.This helped him associate the word with the command as well as the hand signal. Don't forget to reward with mini treats.
To teach Sit: Start by showing your dog a treat. Say, SIT and give the signal with your hand. Make sure he can see your hand command and with the treat in your other hand, raise it above his nose. This should force his head up and his behind should start to lower into a sitting position. If he stands, start over. When he does SIT, say Yes, and reward with the treat. Repeat these steps until he sits quicker. You want to phase out of showing the treat and just use the word and hand gesture. You can continue to reward with treats, but do not show them until he Sits.
This training is truly amazing and you may make up any hand signal for the command you wish your dog to do. I have various tricks that my dog can do with the sign language I have created for him and our family.