I'm often asked how I manage to achieve particular features of my cakes and my answer is invariably that there is a tool for the job! Here's my top items in my cake decorators toolkit:
1) Icing smoothers
2) Tylo (CMC) or Gum Tragacanth and icing glue
Not strictly a tool but an invaluable part of my arsenal nonetheless. The addition of a small amount of either of these powdered gums will transform regular icing sugarpaste into something that will hold its shape when modelled, not get too sticky, and allow you to roll it extremely thinly. You can also buy ready mixed modelling paste but I really don't see the value when you can so easily make it yourself.
Icing glue is simply a hydrated version of CMC and I use a fine brush to stick items together in most of my designs.