ooo! i'm also working on the first one but i'm ever further back on the training; yesterday we started practicing a calm walk with higher set neck (consequently, shorter reins). Got a few ones after a long argument but only on serpentines, not on a straight line) so I have the same question! (sorry i'm of no help)
I did manage one or 2 halts with the energy up where she stood still but i basically froze and stayed frozen as soon as the horse stoped, i was breathing, veeeeeeeeeeery smoothly, but not a single other muscle in my body moved. That seemed to work, though it doesn't sound very practical does it?