I have a wiggle butt too... if I'm not riding right it's like there's a whole other horse back there doing it's own thing.
Like Kayty said... forward. I concentrated too much on halting at X and ended up with a horse that swung out in behind because my aids were thinking side to side not riding back to front.
I do work a lot of straight lines, but different lines. I will ride the quarter track, diagonal, center line, and then lines at random. I work in a hunt ring so I pick a pole and head that way. I work on a trot that is forward and attentive. I've found if I ride like I'm not stopping, we have a better halt. I can stop squarish (won't say square all the time but his tush doesn't point a different direction anymore) almost anywhere I want.
Primo got so used to halting at x that he'd balk going up to it and wiggle and wander about well... yep like a drunk. Now that we trot about randomly halting he is so much more forward, balanced and straight. It's the only part of the test I don't practice.
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse