This article caught my attention about proofing a sit. It is often the first behavior we teach our pets but how well do they really know it. This article breaks sit down into several steps to really get a proofed sit from your dog.
There are so many things you can train a dog, you could easily spend years on end working on something new. Remember, every dog own is a trainer so go out and teach them the good behaviors; not absent-mindly reinforcing the bad behaviors.