Found this great article entitled Speed Bento- Ice Cube Tray Nigiri.
Sushi snobs would likely frown on the use of ice cube trays in making sushi, but if you are trying to prepare a lot of sushi or simply not very good at it, this will save you the time and aggravation of trying to get the nigiri, or rice balls, to a uniform size.
Just put the topping in the bottom of each section of the tray, and cover with sushi rice. I oversimplified that, so please see the original article for more detailed instructions.
And remember! Sushi rice is NOT just white rice! See the recipe below to learn how to make sushi rice at home.