There are several easy ways to fill your cupcake liners with batter; a spoon, a measuring cup, a ice cream scoop, or a pastry bag. You can find examples on filling cups with batter plus lots of other fun tips and tricks on Wilton.com. My favorite way is with an ice cream scoop like the one shown above. When the handle is squeezed it activates a little band that scrapes out the scoop and releases all of the batter inside. I have two different sizes, one for regular sized cupcakes and one for minis. If you watch Cupcake Wars, you'll see most of the competitors use an ice cream scoop. It's just really easy to use and mess free. I bought my scoop at Bed Bath and Beyond.
I think the pastry bag is a pretty genius idea too. I have never tried it before, but think I need to.