I'm curious how everyone trained it? I think there are so many interesting training perspectives on this....
This. Placing mat and toy.I prefer to use a small white towel on the ground with the toy on it to mark the spot instead of using a "tree" that holds the toy and is visible to the dog. Always use the center line of the field when training the exercise. If you use one field consistently, look for a natural marker that is in line with the centerline that you can use in the future to send the dog out further than required. Ideally, try to train this on the field you will be trialing on.
The problem with the tree is that is becomes a cue that will be totally removed at some point. With the towel, you can have it larger at first, and then decrease the size of the towel and eventually fade it out. The dog doesn't learn to look for an object holding the toy. Start out with short distances. This exercise is so much easier to train with a dog with a ton of drive, especially hunt drive. Don't worry about the down until you have a solid send out.
I don't like to put anything on the ground. I've found that when dropping something on the ground the dog just gets used to running a certain distance then they start to search for their toy. This happened to one of my dogs when playing at a different field that was longer than mine. She ran out to the distance she thought it was going to be then started casting for her toy instead of running until she was downed.
I now train it by placing a ball in a fence or on a tree. Something that physically stops the dog. That way the dog runs until it's physically stopped or I down it (this method has worked for SAR as well). I also never practice the down on the send out. I want my dog to run as fast as it can until downed. If I down it every send out then it will anticipate the down.