Cakes usually sink when the cake batter is not completely cooked; everyone’s ovens are different so you need to find the optimum baking time for you oven. Also towards the end of baking test the cake repeatedly to see if it is fully baked. A skewer inserted into the cake does not always mean that your cake is done, this is especially true for very light and airy cakes.
However other things can be the problem, here are a few:
But generally MOST cakes sink/dip because their outer edges were fully cooked but the center was not!
Chef Nicky - The White Ombre (Singapore) - http://thewhiteombre.com - http://www.facebook.com/thewhiteombre