Unfortunately dry food doesnt actually clean the teeth as was once thought, it can actually contribute to dental problems as the biscuit tends to shatter and the resulting debris accumulates around the teeth - plus, many cats don't crunch - they just swallow! so no benefit for the teeth at all. Of course wet food also doesnt do anything for the teeth either, but a diet closer to what cats would eat in the wild, i.e. raw meat/bones, so things like feeding chunks of raw meat and chicken wings, will help keep teeth clean.
You can also add things like Plaque Off to the food which is supposed to help prevent plaque formation, and Logic Gel is also supposed to help.