How can I keep my homemade sushi from hardening?

I make sushi homemade, and I've tried storing it in the fridge overnight. However, the rice would be hard when I would eat it in the morning. I tried putting it in a container, but it didn't work. Whenever I buy sushi and leave it in the fridge, I can keep it there for a good two days. What can I do to keep the rice soft like it is when I buy it? Making sushi is a lot cheaper than buying sushi, unfortunately. And my sushi contains no raw fish, so I'm not worried about any salmonella or food poisoning.