If cakes are not baked for long enough, they will sink. So follow the time stated in the recipe, use a timer, and don't be tempted to open the oven door during baking. Using too cool an oven, or opening and shutting the oven door during baking will cause sinking, as will too much raising agent in the cake mixture.
Cakes are ready when they are firm in the center and have slightly shrunken away from the sides of the tin; sponges should spring back when touched with your finger.
** Asian Recipes