I used to wrestle with clamps and a framing square on every base cabinet, checking each corner twice and still ending up with a parallelogram. Last month I started using a couple of 2x4s cut to the exact diagonal length, tacked across opposite corners while the glue sets. It pulls everything true in about 60 seconds, and I can stack three boxes before the first one is dry. Anyone else use this trick, or do you have a different method for keeping 36-inch uppers square?
Tacking those 2x4s on sounds slick, but I gotta ask, how do you keep the wood from splitting when you nail across the face like that? I learned the hard way that pine will crack on you if the nail goes in too close to the edge, so I ended up screwing little L brackets on instead. Now I just eyeball it with a speed square and hope for the best most days.