The best way to make sure you eat a soft and delicious onigiri, is to eat it on the day that you make it without putting it in the fridge. If you want to prepare it in advance, I recommend freezing it (without the nori) and microwaving it on the day of use. If you wrap the onigiri in plastic wrap, then again in a towel or kitchen paper, it won't get as cold and so won't become so hard.