I used a lustre spray for some of the cupcakes, which just gives the icing a really nice shimmer. I think I bought this at David Shuttle but I'm sure you can get them in any cake shop. It's really easy to use and gives such a professional finish.
You can see the effect of this pearl lustre spray below - it just leaves a really glossy finish, which was perfect as the party was themed black and silver - hence the cupcake cases as well. The chocolate letters I used on these cupcakes I think were just from a supermarket, probably Tesco or Sainsbury's.
On all the other cupcakes where I piped the buttercream, I topped with edible glitter dust which I absolutely love. My friends mum bought these from a cake shop for me to use. They're from the range 'edable art' and are in the colours disco silver, and disco black hologram. The silver is so sparkly and more versatile as it would go with any colour icing and is just more simple and elegant. The disco black is also really pretty however, since it has different shimmers of colour - I would recommend both. They were £2.40 each which seems quite pricey for such small pots, but there's so much glitter in there that you probably would never need to re-purchase them!
Below shows the silver glitter, and the picture below you can see the black with the different colours shimmering.
Hope this has given you some ideas of how to decorate cupcakes to give a classy, simple look - they are perfect for any celebration, especially something like New Year's Eve as well.