A few PRACTICAL words about canine behavior
(and why we watch our dogs' tails)
Canine BEHAVIOR is influenced by a dog's ENERGY LEVEL and his STATE OF MIND. Changing a dog's behavior for the better requires changing both his energy level and his state-of-mind.
The best way to promote a relaxed state of mind is to take a well-paced side-by-side walk together prior to coming to our dog park. On your walk, make calm decisions about pace and direction that encourage your dog to follow your lead. In this way, you're exercising his BRAIN as well as his BODY.
Consider your dog's TAIL to be his "excitometer," since its position tells you a lot about his energy level and state of mind!
Caveat: It takes a little experience to apply this to dogs that carry their tail over their back naturally.
Think of your dog's tail as a clock with one hand. If, when standing in this direction, it points to:

4:00 - he's concerned
6:00 - he's relaxed
7-10:00 - he's engaged
10-12:00 - he's excited
that excitement doesn't
                     equate to happiness.
                     Excitement is just that:

If any TENSION exists in his tail when it's between 11:00 and 1:00, watch out! He needs immediate, calm and matter-of-fact redirection to a lower energy and more relaxed state-of-mind.