The Basics of Good Body Conformation
Balance is important too. The front and rear of the dog need to be in balance with each other and the dog should not be long in the loin area. Make sure the dog has good length of leg, especially below the elbow or it won’t be able to jump. If it is too up on leg (too long) it will be unstable in jumps because its center of gravity will be too high.
Good bone is also important. A very light dog is favored by some but the chance of injury is high. Good muscle and moderate bone makes the best and most athletic dogs. For more information try the book Dogsteps: What to Look for in a Dog by Rachel Page Elliot.
The Sheltie Breed Standard also has a great deal of detail about head structure including head planes, wedge shaped head, finish of under jaw, bumps on the skull, ear placement, eye shape, rules about percent of white color present, twisted or gay tails, that will keep an otherwise great dog out of the show ring. These characteristics do not affect the dogs ability as an agility dog. If you deal with a good breeder you will be able to get a dog with correct structure but a problem in another area that keeps it from being competitive in the show ring.
For performance dogs here is a word about drive and disposition. You do need a dog with brains, drive and a stable personality that can take the stress of competition. That being said a sheltie is not a border collie. A sheltie performs out of heart, because it loves you more than anything and will do anything in its power to serve you. If you want to sit on the couch and watch TV it is fine with that. It does not really care about the show ring or the agility course--IT CARES ABOUT YOU!!! Form a strong bond with your dog and it will move mountains for you on love and its faith in you. Keeping this in mind you must use only positive training methods. Never be negative or harsh with a sheltie; it will shut down and quit trying for fear of making a mistake. Shelties are so sensitive to moods you must be careful about letting your nerves, frustration or disappointment travel down the leash to your dog. You should keep it light and as happy as you can. If you have a chance go to one of Kory Kaye's "Building Speed, Drive and Motivation" seminars. Watch for one and sign up early.