I use to have these problems and here are the things that helped me:
I started using a flower nail in the middle of the cake. They bake up even and i don't waste cake with a dome. With this method i don't usually have to trim anything off my cakes.
I do scratch not cake mix but the cake mix you are using may actually have too much baking powder in it. My cakes were baking up high and then shrinking in on the sides and shorter after cooling. I went from 4 teaspoons baking powder to 3 teaspoons baking powder and 1/2 teaspoon baking soda. This helped alot.
Recipes like strawberry for example were i was adding purried (sp?) strawberries shrink more so I just plan to bake 3 layers for those.
Try baking at a lower temp. I bake at 325-330 for most of my recipes. They bake up even and hold their hight better in my opinion.
And this one may seem crazy but try filling your pans a little less. I fill mine 1/2 full for chocolate and about 3/5 full for vanilla. I think the extra un-supported cake on the top is causing the cake to shrink some as it cools.
If all else fells torte and fill each individual layer to add more height.