I use both pos and neg reinforcment. When she does good she gets a treat, when she doesnt perform up to expectations or completly fails she get a punishment. I dont like punishing her, and i very much like giving her rewards and treats. I feel if you only punish and dont give out any rewards a sub will feel that even if they do good they get nothing. If you only give positive rewards and no negitive rewards you are going to end up with a very bratty sub that wont be responsive to your desires... thats just my 2 cents though i could be wrong..
edit: bah he beat me to it!