Backing didn't work for me. My farrier suggested I basically put her in a one rein stop as I mounted. First I brought her head around on the ground, telling her to whoa, flinging my arm over her back. Then I brought her to the mounting block, brought her head around and leaned into her. If she moved, then she got moved around a bit, then told to whoa. Then we'd go back to the mounting block and try it again. The whole time I'm telling her to whoa as I lean into her, throw myself over her back, etc. I did this both bareback and with a saddle on, putting a foot into the stirrup, then taking it back out again, expecting her to stand patiently.
Finally, I got up on her back, still with her head in the one rein stop position. I kept her flexed that way until I was settled and ready to go. Then I would let her relax for a second or two before allowing her to walk forward. If she moved before I signaled her to go, she got circled.
It took a day to train her to stand until I was ready to move. Now as I lead her to the mounting block, she turns her head on her own and waits patiently for me to get on and get settled.
"When you're young and you fall off a horse, you may break something. When you're my age, you splatter." -- Roy Rogers