A few things you can do.
Have you done any pivoting work, aka the "elephant trick?"
If done correctly, the elephant trick teaches your dog to stick close to your side, whether you're walking forward, back, or sideways.
I taught this to one of my dogs a long time ago, but it was long before I knew about the elephant trick. I used a treat at my dog's nose, with him standing beside me, luring him away from me just a tad. At the same time I stepped into him a little bit(not forcefully, just stealing his space so he wanted to move away from me--but the treat in front of his nose kept him kind of straight). We started out with just a teeny step, click and reward. Once he had the idea, then I'd reward only after two steps. Then three. And so on, until all I had to do was kind of wave my hand in front of him and step towards him and we'd sidestep several steps.
But, lol, I actually had forgotten until a few weeks ago that I taught him this.
Not sure if he'd still do this or not.
The elephant trick is actually a better way to teach this though. Kikopup has a great video on this. I personally kind of combine her way and Silvia Trkman's way. They are very, very similar. I think that maybe with Silvia's way the dogs have to think a little more. I haven't watched Kikopup's vid in a while; if I had time right this second I could explain the differences better...
Anyway basically if your dog understands that they need to follow your leg wherever it goes, then voila....you've inadvertently taught your dog to sidestep, heel in reverse, weave in reverse, and various other backwards tricks all while improving your heel.
Hope this helps. If you need more details feel free to ask.
Here's Kikopup's vid: