Stay means STAY

Practice some heeling a sitting, throw a down in there occasionally.

Heel, come to a stop, and 'sit' your dog

Say 'STAY' and use a hand in a 'stop' gesture in front of your dog.

Step forward on THE FOOT OPPOSITE YOUR DOG, and turn and face your dog.

Begin adding some distractions.
Initially, start small with some movement from you:  Take a few steps to the left or right. Circle your dog. Try Jumping up and down.

Return to heel position. REMEMBER: the stay isn't over until you say 'Heel.' If the dog moves at any time during this exercise, repel up the leash, lift on the leash and place them back in the sit in less than 3 seconds. Channel your inner ninja! You must be quick.

Next try some harder distractions: Throw treats, let other dogs run by, introduce cats, throw treats, run the vacuum cleaner,
open and close doors, flip some burgers on the grill…anything you can think of that does not actually 'call' the dog 
away from the position.

You must stay within 6 feet of the dog, and the dog must remain on leash so you can act quickly and correct if needed. Do NOT repeat the sit or stay commands!

Practice the above exercises each day until you have 10 refusals to break with each
distraction (10 uncorrected stays).