Don't be shy with your frosting. It is a good idea to use a large blob of frosting from the start and spread it out to the edges of the cupcakes. This way you can glue down all those pesky crumbs and keep them from getting into your smooth frosting finish. You can always shape the top and remove any excess frosting after you have the entire top of the cupcake covered!
We use an offset spatula for spreading the frosting. They come in a variety of sizes and the offset angle built into the blade keeps your fingers out of the frosting while you push, pull, and swirl the frosting. Offset spatulas are available at some grocery stores and baking supply stores. If you are having troubling finding them try the AMAZON link below for an on-line source.