I got my black lab at 6 months old and he loved to fetch a tennis ball and would bring it straight back and drop it at your feet however, after living with 2 small children for a few months he became quite possesive about his tennis balls and refused to give them up even though he knew this meant not getting them thrown for him.

At first I had some success by trading him the ball he had fetched for a second one, but this was still less than ideal. I have been having better results though since I stopped using tennis balls, I now use a variety of toys, including good old sticks, for our fetch games and he seems to be less possessive of them and more interested in getting them thrown for him to chase. Balls on a rope are good and easy to get some distance with but his current favorite is the Kong Wubba. Once he has chased it down I let him have a run with it before calling him to me which usually does the trick. I only claim the item once he has come to me and if he doesn't drop it on his own. I have also found that giving him something else to do before each throw helps, something that he does reliably like sit, therefore when he comes to me with the wubba in his mouth and I tell him to sit, he drops it and sits down ready for the next throw.

Hope this helps