Piping frosting onto cupcakes, instead of simply spreading it on, can really change the look of your desserts. With neat little swirls, the presentation will look a cleaner and more professional. I have a pastry kit and will use the various star tips from time to time, but the best - and by far the easiest - way to pipe on frosting is to use a regular plastic bag. Simply scrape the frosting into the bag with a spatula, twist the bag at the top, just above the level of the frosting (this pressure forces it to go forwards, rather than getting stuck in the bag) and snip off the corner. A large opening is the best for covering large areas, but a small opening can let you get really precise when you want to use this method to ice cookies or other intricate patterns.
Kids love to use bags like this for decorating, by the way. They are much easier to maneuver than a spatula full or frosting and usually end up making less of a mess. Use a ziplock bag for an extra barrier between the frosting and the floor, just in case.