Start in an area with few distractions. Have your dog in front of you and get his attention with the treat. When he's looking
at the treat slowly move it over his head toward his butt, as you say "sit". He should follow the treat with his
nose and automatically put his butt down as his head goes up. (In some cases the dog may back up, in that case I just shorten
the leash so he can't, once he gets the idea you can gradually give him more slack). As soon as his butt hits the ground give
him the treat and an excited 'Good Boy'. Practice this for a few days.
Once your dog is responding well to this start giving him the treat every other time but keep giving him the verbal praise.
After a couple days of that really change it up a bit, do three or four sits before giving the treat, then do every other
sit. Your dog will never know after which sit he'll get the treat, the benefit is you won't have to give your pet a treat
all the time. After training your dog will still give you a good sit, even without a treat. Every once
in awhile you will still want to give your pet a treat just to reinforce his thought that 'Hey I never know if they've got
a treat', that should be enough to keep his performance good.
The bonus to training with this method is that while you are teaching your dog the verbal command you have also taught
him a hand signal for sit. To try it out just put your thumb and forefinger together and move your hand from in front of your
dogs nose toward his butt, just like you in the beginning with the treat, he should sit.
The only thing you should be careful of is what you do right before you lure your dog with the treat and give the command.
I once inadvertently trained one of my dogs to sit whenever snapped my fingers. There were a couple times I snapped to my
fingers to get his attention and then told him to sit. After that if I'd snap my fingers he wouldn't wait for the sit command,
he would just automatically sit.I actually liked the way my accident turned out, but you might not.
I would also like to add that this method will work dogs of all ages, I have used it on puppies as young as four weeks