Use a heavy enough cutting board with enough real estate (size) to work on. I like to use a cutting board in the 12"x18" size or larger. Don't use a flimsy cutting mat. Another trick is to put a damp clean kitchen towel under the cutting board for stability. Also, if you're dicing a lot of tomatoes, use a cutting board with a juice groove to contain the mess of the juices from over flowing onto your counter.