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After your dog beings consistently pawing at the treat in your closed hand, you can start to introduce your preferred verbal command. Wait until your dog paws at your hand and issue the command while giving him the treat. Your command could be any word, but "Shake" or "Paw" are commonly used. Say your command clearly and loud enough to be heard by your dog. Issue your command at the exact moment the dog paws at your hand. Once you pick a command, don't change it, as this will confuse your dog. Keep any command short. Generally only one word commands will be the best. After you have begun using your verbal command when your dog paws at your hand, it's time to start saying it before it paws. As you move the hand with the treat towards your dog, say your command. This step helps him to realize the verbal command is now the signal to bring his paw up to shake. Ideally, your dog will bring his paw up as soon as you say your command. Only after he shakes should you reward him with the treat and praise him. If your dog doesn't bring his paw up at the command, try again until he does. If he still doesn't after about fifteen minutes, stop for a while and try again later. You don't want to frustrate your dog. Rewarding your dog for any other behavior will confuse your dog. Never reward him unless he has completed the command successfully, or else he might view your rewards as bribes. Avoid improper rewarding by always obtaining your dog's full attention before training. Don't get frustrated and give your dog the treat if he isn't doing the "shake" command as you asked. Giving up like this will send the message that if he sits and ignores you he will be rewarded. Realize that your dog is always paying attention. Any treat given to him will likely be associated with whatever he was doing at the time. Your dog wants to earn treats. Once he makes the connection that a behavior will earn him something tasty, he will be ready to behave in that way. This counts for both good or bad behavior. Be aware of this when you reward your dog.
Introduce a verbal command. Start preemptively using your command. Reward your dog only when it completes the command.