What's the best way to store unused sushi rice?

Storing cooked rice is generally not a good idea.

With the high starch content in rice, it is simply an excellent growing ground for bacteria. Add to this the fact that uncooked rice can contain spores of Bacillus Cereus which when germinated to bacteria can lead to food poisoning causing vomiting and diarrhea.

Bacillus Cereus spores can survive cooking, so when rice is left to cool they can germinate and start multiplying in the rice. It isn't the bacteria itself that is dangerous, but a toxin they produce. This toxin does not disappear from re-heating.

If you do want to store the rice, make sure to cool it really quickly and store it in the fridge for no more than a day. Personally I never do that and only cook as much rice as I need.

Anecdotal: In fact, a friend working at a big sushi chain in London told me recently that over the course of four or five years they had one incident of proven food poisoning in one of their many restaurants. The source of the food poisoning was not the fish, but actually the rice which had been standing too long.

Storing nori I don't know much about, but if you seal it in an airtight bag and keep it dry I see no problems with that.

I don't agree with Manne on storing rice. If you refrigerate what you don't use, then long grain rice could be used for a rice salad (similar to pasta salad) or fried rice. You might be able to use short grain rice in a rice pudding.

But once the vinegar and sugar is added to make sushi rice -- I wouldn't try saving it for sushi -- it's not going to have the same consistency the next day, and I really don't think it would work. You'd be better off making a vegetable roll or two with the leftovers and chilling that for the next day.

Off hand, I'm not sure where I might re-use leftover sushi rice where the consistency wasn't an issue ... maybe a rice-based casserole?

As for the nori -- I use a zip-top bag, and compress the air out, and haven't had any problems.

For a different take, I've stored seasoned sushi rice (Nishiki) for a few days in the refrigerator, reheated it in the microwave and had acceptable results. The consistency isn't quite as uniform, fluffy, or cohesive, but it can still make a decent dinner with a couple ounces of fish from your monger.

Regarding food safety, and speaking as a biologist who cultures microorganisms for a living, cooking the rice (boiling) will kill the vast majority of bacteria. Not all, but the only cooking method that really cares about true sterility is canning. Unless you leave your rice in the danger zone ^{(not that danger zone, or this one)} for an excessively long time, the amount of bacteria that survived (few), wake up (fewer), and reproduce will be negligible.

Basically, buy a refrigerator thermometer and make sure you're keeping it cool enough (2-4 °C, or 35-39 °F). Not just for this, but for all your food.