Don't get a shock collar, those things hurt the dog. Just continue to train it and things should get better. Go with what Ss2k3 said.
And if you doubt me that those things hurt, just try one on for size...trust me it hurts like hell. To prove a point my buddy and I tried it to get his step mom to take it off of his dog.
Holy s*** is all I can say. We kept trying to get it to go off and it wouldn't, all of a sudden the damn thing went off and shocked the hell out of us (it was one of the bark collars, so just picture me and my friend barking, trying to get shocked).