To make nicely formed onigiri, the key is applying the right amount of pressure. That, and keeping the dang rice from sticking all over your fingers -- more this technique later. Personally, I prefer to simply use a plastic onigiri mold that I picked up from Uwajimaya. Flavorwise, I tend towards classically salty and savory items, but you can experiment - even see what's leftover in your fridge and see what works best for you. So, in short, there are lots of Onigiri variables: (1) the shape of onigiri, (2) the filling of onigiri vs. flavoring distributed throughout the rice, (3) to wrap with seaweed or not and so on.
Onigiri is pretty ubiquitous in Japanese convenience stores, where they come in these amazing little plastic contraptions to keep the seaweed separate from the rice (otherwise it would get too soggy). You can also find rice balls in other Asian cuisines, like Korean, where they are called jumok bap or the ever popular Hawaiian SPAM musubi, a particular variant of rice ball with flavored, fried SPAM.
Serving Size: 2-3
Preparation Time: 30 minutes