A spray bottle filled with water. Our little beagle is so stubborn, but when she's being naughty (barking excessively or chewing on something she's not supposed to be chewing on), a quick squirt of water stops her in her tracks. Actually, we don't even have to spray her anymore. She just sees the bottle come out and stops.
Also, I highly recommend pet insurance. It's been a great thing to have for both of our dogs. My beagle recently had a stomach virus where she was very dehydrated and had to get all kinds of meds and fluids. It cost me $600 but with pet insurance, I got $450 of that back. I use VPI, by the way.