Inflexible plastic drinking straws placed evenly in a circle on each layer that will support another layer ensures that the upper layer will not sink. This was a brainstorm of my husband, Elliott, who assured me that straws could withstand a significant amount of weight without displacing nearly as much cake as wooden dowels, which are much more difficult to cut to size as well.
To insert the straws, first mark an outline on the frosted cake tier of where the next tier will be placed. You can use the pan the layer was baked in as a guide by inverting it over the center and touching it down lightly to mark the frosting. Insert a plastic straw into the center of the cake until it touches the base and use a pencil to mark off on the straw the height of the layer. Remove the straw and cut it off at the pencil mark. Use this straw as a guide to cut other straw supports the same length. (If the cake will be assembled where it will be presented, you can cut the straws ⅛ inch higher than the cake to prevent frosting from sticking to the tier above and make serving easier.)