Stay Basics
Although the position you leave your dog in will vary depending on the situation, there are some parts of the stay that will be the same every time. The release from the stay will be the same every time, as will the command.
The Stay Command
Like the release command, the stay command is a combination of a verbal and a hand signal, although you won't actually touch your dog with the stay signal. The command should always be clearly and decisively a command, not a question. The verbal command “stay” is given one time like you mean it, in combination with your right hand, open flat a few inches in front of your dog's face.
In the early stage of teaching the command, you'll always be leaving from heel position, with your dog on your left side, so the leash will be in your left hand, leaving your right free to give the hand signal. Don't have treats in your hand when you give the stay signal!
Should I use treats to reward stays?
It depends on your dog. Some dogs are too distracted by treats to focus on the stay, but if you can reward your dog and remind her to stay without her breaking, you can use them to reinforce maintaining the stay. If you use treats to reward stays, remember to always reward before the release, while your dog is still in the correct position. The treat is for holding the stay, not for breaking it.
During the Stay
After you've given your dog the stay command, the responsibility for staying is his. You'll correct if you need to, but don't do all his work for him by trying to hold him there with your hand extended, chanting “stay, staaaaaay, stay.” If you pay attention to your dog, you'll quickly learn to read his signs and interrupt him while he's thinking about breaking the stay.
In any case, you will correct him if he does break the stay, but he's responsible for staying. Let him do the work during practice. Think about it this way: how you teach it is how he's going to know it. A stay he won't hold unless you're constantly reminding him to isn't very useful for you, and you're doing this to make your life easier!
If your dog doesn't lose it with praise, you can quietly praise occasionally, “Good stay,” but don't pet your dog during the stay. If your dog has reasonable self-control in the presence of food, you can also slip your dog a treat occasionally with praise, and a gentle reminder, “Stay,” to help build duration.
Clear and Reasonable Expectations
You should have a clear idea about what you expect from your dog when you give him a stay command so you can communicate it clearly to him. If you expect him to hold his place and position until you return to him, make sure he does during practice. Reliable stays take lots of repetition, and are dependent on building on success. Be realistic about the amount of practice you've done before giving your dog a stay command in a novel situation. Don't sabotage yourself, your dog, and your training program by using the stay command thoughtlessly, like to tell your dog to stay in the house when you're on your way to work.

