"Sit" (for puppies)
Use the treat to guide your puppy into the sit position (treat just above nose, moving slowly towards the rear. He only gets the treat when his butt hits the floor (no treats for jumping up).
When he sits, pop the treat in and say "good boy" in a calm but happy tone. You want him to hear that praise phrase whenever he gets a treat. (i.e. good boy=treat)
Repeat this process 20 times. He should give less and less resistance and should be sitting quicker each time.
Next, ask him to sit using your hand signal (a treat in your hand just over his nose). If he sits, praise and treat. If he doesn't sit, pull up GENTLY on his leash or collar and push down on his rear so that he sits. This is how you will enforce your command if he chooses to ignore it.

Sit-stay:

Once your dog knows the sit command, ask him to stay.
If he gets up from the position, use the same correction to enforce it. (pull up on the leash, push down on the butt). He should hear "no" as you are correcting him. When he's in the sit position again, you can ask him to stay.
At first, just demand that he stay for a few seconds. When he has done that, you can release him from the command by saying "okay" in a happy praising voice. You may have to take his leash and lead him a few steps so he knows he can move.
As you and your dog practice this, add a few more seconds to the amount of time that he stays. Release him if he makes any improvement. Set your first real goal at 20 seconds. Then go further, but go slow and don't expect too much too soon!

When your dog can sit and stay for 15-20 seconds, start adding small distractions, such as your taking one step away. If your dog breaks your command, you must correct him and get him to sit-stay again. Try these tests, one by one and remember to go slow! -
Get to the end of the leash without having the dog break.
Circle the dog while it remains in the sit. The hardest part for the dog will be when you go behind him where he can't see you. Be ready to correct.
Go to the end of the leash and squat down to make him think you want him to come over to you. Don't call him, just use your body language to trick him. Correct him if he breaks the stay.
Jump up and down and make silly noises.
Jump around him and make even sillier noises.
Place a treat on the floor about 10 feet away.
Gradually make the treat move closer and closer.
Throw a treat across the floor.
Throw a ball or toy across the floor
Make him sit/stay for food.
Have someone ring the doorbell.
Have a person turn the doorknob. You can also do this yourself if no one else is available.
Open the door slightly and SLOWLY!
Open the door even more in increments until it is open all the way.
Touch the screen or storm door handle.
Jiggle the handle as if you were about to open the door.
Open the door slightly.
Open the door more and more until it is open all the way.
Release the dog and go just outside the door. Then make the dog sit stay again right away.