Mixing the biryani is a technique that it learnt with practice. Do not mix all the rice with the sauce in the roaster at one go. We mix and serve as much as we need, this way the biryani remains hot. But if we have to store the biryani to eat later, it is better to mix all the biryani in the roaster after resting it for 10 minutes, otherwise the rice gets packed and dry and the sauces do not get absorbed in the rice that well.