I would say the biggest thing is to use color shaping tools to help you smooth out your sculpture as you are working on it.
After that you can use clay softer, rubbing alcohol or even baby oil to make your sculpture look even smoother and it also helps to remove finger prints
Also, instead of applying the clay right over rough aluminum foil you always buy Sculpey ultra light instead. That way you way you will always have a smoother surface to apply the clay to