If your dog has mastered the "tug" game and the "bring it" command, then teaching him to fetch will be easy. Fetch comes naturally to some dogs, and to others it is a moderately easy trick to teach. Once your dog learns Fetch, he will enjoy doing it not only for treats, but simply because it's just a fun game. Follow the steps carefully with your dog, and soon you will have another fun game to play with him.
Step 1: Start by playing tug. Say the "take it" command when you want your dog to pick up the rope toy, and then the "give" when you want him to release the rope. Step 2: Have your dog "give" the rope, but then toss it a few feet away. Say, "take it." Click treat when he picks up the rope. Step 3: Generally, your dog should be excited about playing the tug game, and will usually come back to play some more. If he doesn't, repeat step two, this time getting him to bring it to you with the "come" command. Say "Give" and click and treat.
Since Caspian is part retriever, fetching came naturally to him. We would throw a tennis ball, and Caspian would be extremely excited—he knew that this was going to be a fun game. He would run down our hallway as fast as he could, and puppy-pounce on the ball. But then, he would get distracted, and not know what to do. He would chase after the ball fine, but he just wouldn't bring it back. We worked with him using the "come" command, and by clicking/treating, it didn't take him long to bring the ball back every time. Now playing fetch is his absolute favorite thing to do!
My dog has learned to fetch, but he always brings it back slowly. Three things: The first thing is to use great rewards. Yummy snacks, great praising, a nice walk...All these 'treats' are good to reinforce the dog's behavior. The second thing is your enthusiasm. If you're not at all excited about the game, how do you expect your dog to be? Go all out; be silly. If your dog sees you enjoying it, chances are he will enjoy it, too. The third thing is time. The more you reinforce this trick, the more your dog will learn to appreciate it! Keep positive and practice it several times per day. After a while, he will learn to love fetch and look forward to playing it.
Tip: "Practice fetching different objects and at greater distances. Be generous with praise and click and treat when he performs well."