To achieve more level layers when making a layered cake, divide your cake batter among the pans using a measurement cup to split the batter up evenly and -- this is going to sound funny and it's a little noisy, but -- level the batter by lifting the pans several inches off of the counter top and then dropping them. Do this several times. This will spread the batter out and bring air bubbles to the surface, which help to make your cake layers bake more level. When you see the bubbles dissipate, you're ready to put the pans in the oven. I swear this works! I have even seen Miss Paula Deen herself using this! Give it a try next time you bake a layer cake.
Did your cake collapse in the center, especially in a 9 x 13 inch pan? This happens to me and it's usually due to un-even oven temperatures, resulting in under-cooking in the center, thanks to my old oven, which will not hold a temperature correctly. Besides using an oven thermometer, try not to peek at your cake until it is almost done. This will cause a drop in temperature and could lead to a collapse once you remove it, so wait to check on it about 5 minutes before the earliest cook time.