The trick is (not that it's a trick) to think of the halt as a forward movement rather than stopping. The minute you think of halting as pulling on the reins and blocking forward movement, that's the second that your horse halts crooked and heavy on the forehand. Halting, in my opinion, involves walking the horse up into the bridle by stretching tall, stilling my seat and closing my legs ever so lightly to get my horse to step up into the bridle with his hind legs. With the reins, all I do is concentrate on keeping my elbows soft to make his head has somewhere to go instead of hitting a wall. If you do it right, you pretty much don't need reins. It's all in the seat. Hope that makes sense.
When I was teaching my horse to square halt, I used a dressage whip. This is what my instructor got me to do: Ride trot-halt walk-halt transitions, using a balanced and deep seat. After the horse had stopped, feel which leg was out of place (eg: if the hind left was behind the other legs, your left hip would feel 'lower') I would then reach the crop to Candy's hind left and give it a tap, while maintaining contact with the reins. She learned to place her feet evenly.
If your horse is travelling forward and straight, it will give you a square halt as it's weight is evenly distributed. Obviously a horse on the forehand is going to 'fall' into halt and leave a hind leg behind. SallyRC, yes using a dressage whip to 'tap up' a hind leg is used frequently. And yes, it will work to an extent, but having the horse forward, balanced and into the bridle is the most effective method of achieving a square halt
Yeah, I always use my leg in the halt, to have my horse lift himself up into the halt, rather than dragging his legs into it. I normally work on trot-to-halts while warming up. It helps my orse listen to my aids and balance himself. He also benefits from this by working his hind end more, and with a naturally short stride, using halts, and transitions gets him to lengthen a little bit.