Backing up is really hard for dogs. I often start by having a wall on one side, and a barrier on the other so it makes a little channel for the dog. If he can't go forward, cause i'm there, and he can't go sideways, the only way for a dog to go is backwards. You can either free shape it by waiting for your dog to offer one paw going backwards, or lure it by walking into him. Just reward the tiniest motion in the beginning though. Gradually start moving your barrier away, and then do it away from the wall. If you're walking into them, get rid of that too so the dog can back away from you.
You can also try placing the treat between your legs so the dog has to back up after eating it. Then click the back up and throw the treat between your legs again!
If you're trying to walk into them and the dog sits you can try holding a treat in front of you at nose level. If your treat is too high it makes them want to sit.
I've also worked on the dog walking backwards between my legs, and then backing up to get to my legs. I reinforce heavily for the dog standing there, then take a couple steps backwards. Once they can do that, click for backing up with me and then throw a treat 6in in front of me. When the dog eats it, he should back up to get back between your legs since that's a reinforcing position, so i click that backing up and throw it in front of me again, gradually throwing it further and further away. Good luck! I've also seen this video on youtube before that i like