From Jill Hamilton, (jillh@VNET.IBM.COM)
Here is a method that encourages the dog to actually think about its path, and so is fairly successful. You can use this technique for both go-outs and recalls. Many Shelties do like to put that herding arc into things, even though they end up in the correct place.
Start with physical guides and teach the dog to stay within them. You could start with a chute of jumps, or use the broad jump boards, which work well as they define the chute but don't physically compel the dog. YOU train the dog to stay inside. You can also use high jump boards laid on the ground or PVC pipe.
Once the dog understands staying in the chute you fade the chute so there is less and less physical restriction, but the mental one remains. Meaning, you taught the dog to stay in and just because the chute is now 2 ropes instead of a row of jumps, it still has to follow the path. This makes the dog THINK about straight instead of just relying on physical obstruction. Instead of the rope you might replace a solid line of barriers with a section of chute every 10 feet or so, so you are still fading the chute.
Continue to reduce the size and obviousness of the chute; broad jump boards can be replaced with 1 X 1's. You can go from painted to unpainted lumber. During the whole progression you are asking the dog to think about where it belongs and teaching it to concentrate on the exercise.
Arcing frequently has a trigger point. If you can get a dog past a certain point straight, it can complete the exercise straight. This is frequently seen with go-outs. Many dogs will go straight until they reach the jumps and then arc, but if they leave the line of jumps straight they can complete the go-out straight. In such a case, a set of guides placed between the jumps may be enough of a reminder to get the dog thru the center of the ring without veering. You may find you can reduce your recall chute to just one set of 3-foot guides placed strategically.
Ultimately you can fade your chute to something really inconspicuous such as folding carpenter rulers. These can grow or be shorten as needed. As the guides become harder to find the dog has to actively seek them to be correct. He has to concentrate on the recall and is likely to be much straighter than when they just put the body in drive and the mind in neutral.