However, if I pulled the strings taut at my sides, on the same line as the back of the pants, then snugged and tied the knot in front, they tended to stay tied.
Recently I've been going a bit higher on the belly and a bit tighter with the gi pants tie - the extra pressure from the belly really keeps the knot tight.
Had the same problem with with my belt. After my first seminar, where re-tying it every few minutes for 6 hours got really annoying, I found this:
This knot does not
loosen and will not
come undone, at least I haven't been able to make it happen!
I switched to this method of knotting the belt ~2 years ago and never had the belt come untied since. My gi jacket does tend to ride up a bit more with this type of knot compared to before when using a square knot.
With a hakama the jacket rides up less but I still notice it a bit and adjust as needed.