What I did with my mare, was get a longer lead rope, and start with things relatively close, but still would have to take a step or two to get to them. Every time my mare would move, I'd put her right back where she was, and then walk again to the grooming box or whatever I was getting, after a bit of this, she stopped trying to follow me, so then I'd start moving further away, but still holding the end of the rope, so that way if she decided to walk off in the opposite direction, I wasn't having to walk after her, and thus encourage her to keep walking even if I wasn't planning to. After she was standing still for that, I would then drop the rope, and continue doing all the grooming and whatnot, slowly lengthening the distance I would go, til I felt confident that she wasn't going anywhere. Then I would start working on the me disappearing from view, which was the worst for her. I would keep a hold of the rope initially, just because I didn't want her actually running off, if she got a mind to, and I'd disappear into the tack room for a minute, grab a brush, come out, brush her, then disappear back into the tack room, and put brush away. Then I'd drop the rope, but where I could still see it if it started moving, and start spending a little bit longer in the tack room. She has learned pretty well how to stand ground tied, though she does still get a bit distracted sometimes, so I have to remind her that I want her to stand still.