@AJs:
Body language is important. Learning how to use eye contact and tone of voice is important too. For instance: If you speak to your dog in a high pitched tone, it indicates excitement and that your are pleased with them…If you lower your tone, especially when they've done something you are unhappy about, they recognize this easier than the words themselves. At least that's been my experience.
Dogs (and cats) use so many communication methods...it's a matter of working with your own dog (or cat.) There are other subtle methods I use and AJ seems to respond to my cues rather well. Again, I have different requirements than other pet people.
Zest's person could probably help too, having looked at her agility trials.
Ah yes, they really do respond well to a higher tone and lower tone when speaking to them. Whenever Kananga has done something naughty, I don't even need to motion anything, I just say his name in a low tone and he instantly responds to it knowing he has done something wrong. The ol' whippet ears go into action.
I always use a higher tone when he's playing so he understands the difference. Seems to work.