2:1 ratio icing sugar to butter, don't add the icing sugar in one go as it could end up to 'claggy' not fluffy.
A tip from a friend who had a cupcake business was to add a drop of water to the mix which results in the icing setting very slightly on the outsude (easier to transport).
Lemon or lime juice, coffee melted in the water, vanilla essence etc are alternatives.
If making a cream cheese topping, add some butter to the cheese and icing sugar otherwise the cream cheese/icing sugar ends up as a dripping mess
as I found out!