Soak 1 cup urad dal and 2 cups idli rava overnight. Grind the urad dal first and then add the rava as well and grind it for a minute or so. Grinding the rava along with the urad dal results in softer idlies not only on the next day, but also when u make them after a few days with the same batter. Also add salt accordingly along with a pinch of baking soda and let it ferment overnight. Better fermentation helps in a better taste of idlies. If you have a wet grinder you can use idli rice as well. Also wet grinder helps in making the idlies softer. Try using the husked urad dal or if not availab