For what it's worth (I'm only squatting between the 130kg and 140kg mark so far, depending on squat style, and hence would generally - and rightly - be considered a rank novice, so yeah, take my advice with as much damn salt as you like

) ...
I've found that the best way to get squats (and other big lifts) heavier - with acceptable form - is to just stick with a basic linear progression squat-push-pull-the-fuck-out-of-it program, squatting heavy at least twice a week, or more likely three times. Keep adding weight to the bar - little bits, is best. And if you stall? Deload by around 10% or so, as of the next session you do that lift, and just resume the small increments as normal. Soon enough, you'll be back up to where you were, and probably over that mark.
If that doesn't work, when you're squatting less than, at a guess, 120-150kg (maybe depends on body weight and stuff), then it has to be either form, diet, recovery (e.g., sleep) or attitude (i.e., deciding to be weak).
No expert, like I said, but that's what I've found to be true.
Then, once the linear progression became
really taxing (couldn't go any further, it seemed), I added in some high rep work for all the major lifts, as well as only squatting heavy twice a week. For me, this has worked. Can't guarantee it would for you, at that stage.
edit: I don't use a belt, so I wouldn't be able to "help" you there, if I can help you at all, lol.