Shampoo hair frequently. Avoid heavily fragrant shampoos or oils. Cut down on junk food, salt and spicy food. Show a Dermatologist and make sure it is not a dandruff mimicker like sebopsoriasis etc. Go through my health feed on practo on Understanding Dandruff. Have a good regular hair care regimen and you can keep dandruff at bay. You do not have to wait for dandruff to start or aggravate. But take proper care so that it doesn't trigger off.