My husband's big booming "NO NO" is usually enough...And then he picks them up a minute later, and in his sweetest voice tells them, "if you do that again, I'm going to spank your furry little butt!", though he never would
I personally use the "Ttsssst!" noise that the Dog Whisperer uses, since my "NO NO" isn't as forceful as his
Also, we use the "Scruff" on the back of the neck-just pull firmly, but not hard, enough to get their attention when they're really naughty. Usually they know they're naughty, and before they get scruffed, and they will both submit immediately and roll on their backs, as if to say-"I know, I'm naughty Mommy. I'm vewy sorry, and I will twy not to do it again. Pwease wub my belly and don't be mad!" It's so cute, I can hardly resist!