I steam idli's using cotton cloth which enhances the softness.The cloth material that I usually use is thin dhoti cloth which is best suited for steaming idli's.
The batter can be put in the fridge without adding salt and we can add salt when we keep outside for fermentation.
Usually aval or steamed rice are added for increasing softness.I added only rice and dhal which gave me perfect idli's.
Usually I add a fistfull of crystal salt for 4:1 ratio.