Soft idlis mostly depend on proper fermentation. Better the fermentation, better for the idlis. But before that, grinding the Urad dal to a fluffy consistency with as little water as necessary is of utmost importance. A wet grinder is perfect for this, since in addition to grinding the batter well, it also helps to incorporate air into the batter which aerates it to make softest and fluffy idlis. While preparing idlis, make sure to bring the water to a boil in your pressure cooker before adding the idli plates.
See more points with pictorials here in this link : How to make Soft Idlis