I recommended bracing each panel every 18" with a 1x2 that ties opposing panels together and lightly preloads them. Most people cross-brace the front and back with two braces, high and low in the cabinet. Cross-brace side to side in the same fashion.

Another common way to brace is to use a square section that spans the entire cross-section, but with large holes drilled in it. There are lots of good ways to brace your cabinets, just make sure that the end result is long panels are rigid enough that they sound like solid wood when rapped with your knuckle.

You might do a search here for "brace" and see what you come up with. Several people have shown photos of their cabinet's internal bracing.