I have taught my labs to trail on command in about three days each. I followed a technique which I read about 15 years ago. I take a piece of fat from a steak and drag it across my uneven yard; about 1/2 of the way across I make a 30 to 40 degree turn.
I then hide their food dish (supper included) behind some cover. I always go downwind. I then go get the dog, who has not seen this, and bring it on lead to where the trail starts. I let it lick my fingers as I lower them to the start of the trail and say 'find it' as I release her. Grandmother, daughter and niece each was able to trail on command after three nights of this. The grandmother would trail at absolute full speed on a wounded pheasant. Here are two examples of the success of the training. I have handled her to 300 yards, blown a sit whistle, and then yelled 'find it' and she would start at that place to trail a bird which she didn't see go down. I have seen her follow a running pheasant from that spot to the far end of a huge field and return with the bird. I was carrying my Tri-tronics collar when I walked in to a Lake about 250 yards through the woods. Because I was also carrying bumpers, I accidently dropped the collar on the way in, and as I sometimes have done, I automatically trained without it. I got back to my truck and finally realized that I had dropped the collar. I took her to where I had started into the woods and told her to 'find it'. She trailed me on the twisting route through the brush on the way toward the lake, and then when I saw her nose go down, I walked over and picked up the collar. This training has had no negative effect on running amateurs and opens, where she got two second places in each.