That old timer at the scrapyard fixed my workbench sag with one look
Guy in Spokane saw me hauling a warped 2x6 top and said 'you're bracing the length, not the sag'... swapped my center support to run diagonal to the front legs, cost me $3 in scrap angle iron and now that bench doesn't flex when I lean on it. Anybody else had luck with diagonal bracing over the usual crossbars?
That "bracing the length, not the sag" line is exactly right, but the real trick is adding a cross brace on the back edge too, stops the whole frame from racking sideways when you plane boards.