Back Up

Method A

Step 1: With the dog standing facing you, bend down quickly and gently push the dog in the chest so they take one step backwards. Say "back up".
Step 2: Click and reward.
Step 3: Chances are, your dog will catch on to this very quickly and after a few "pushes", you will be able to just slightly bend over and "pretend" to push them, saying "back up" and they will take a step back to avoid the push.
Step 4: Substitute in the fake push for a "shooing" hand signal and repeat.
Step 5: Gradually require the dog to take more and more steps backwards before the click and reward.

The above method is provided by Gio FM

Method B:
Step 1: Have your dog sitting in front of you, facing you.
Step 2: Walk towards your dog. You dog will naturally try to move backwards because you are getting too close to his space.
Step 3: The moment your dog moves backwards, say the word "BACK UP" (or click) and give your dog a treat.
Step 4: Practice and gradually have your dog to back up for a few more steps before saying the word BACK UP.

Method 2 is provided by Mini

Method 3:
Step 1: Stand with your legs apart.
Step 2: Put a small treat on the floor, in between your legs.
Step 3: You dog will go forward to get the treat, then the dog will usually step back to look back up at you.
Step 4: As the dog steps back, Clicks and put another treat on the floor in between your legs.

Method 4:
Step 1: Make a narrow channel, that has only 1 way in.
Step 2: Led through it a couple of times first without the end being blocked to make them comfortable with it
Step 3: Block the front end of the narrow channel.
Step 4: Stand outside the channel and lead your dog up the channel. Since the front openning is blocked, the only way out is for the dog to back up.
Step 5: Once your dog backs up, click and treat.

Method 3 & 4 are provided by Bertrum, edited by Mini