The secret to a proper layer cake?

All of the above, plus practice makes perfect. The more you do it, the more you'll learn, and the better your cakes will turn out.
I also rely on friends, family and neighbors, who are happy to sample and offer suggestions.

Two things come to mind here. A "crumb coat" is essential - brush loose crumbs off as well... And insulated strips you apply to the pan before baking. They can be found in a cake decorating section of hobby/craft stores. They are silver strips/pads that you soak in water, wring out, then put around the perimeter of the pan before adding the batter and baking. The insulate the outer edges so it cooks slower and you have less of a center rise. The finished cake will be close to level! If it isn't, trim it down with a slicing knife before crumb coating.

I'm a home cook, and no professional... However, I do have a cake cutting device! It's a piece of shaped metal with notches up the sides, with a thin wire that runs across. You can choose the thickness to cut your cake on the notches, and then run the wire through the cake. It provides even layers, so you don't have to try and stack wonkily cut slices back together to make a straight/flat cake! It cost about a fiver ($7) and was well worth it! I also second the crumb coating... Thin coat, in the fridge for a bit to set, then do a proper layer of icing. I also find it helps to put plenty of frosting on, and then take it off, rather than find you haven't put enough on and then want to keep adding to a thin bit.

Also! If you don't already...Hot spatulas... A mug of hot water to leave it in, wipe on kitchen roll, and smooth!

Just a few days ago on Food52, there was a featured tutorial called Mastering Layer Cakes. In the search box on the home page, upper right hand corner, write in LAYER CAKE and choose FEATURE from the drop down menu. Sorry I can't copy the link on my phone.

While everything Maedl says about professional food photography is true, professional bakers have their own tricks to make cakes look perfect too, like the "crumb coat" described. They also know how to use a cake turntable to create even layers. It's all part of the art of baking (as opposed to the science of the craft).

Take a look at the books published by Rose Levy Beranbaum for recipes and techniques.

Food photography can lead to exceedingly high expectations for home-cooked food! Keep in mind that first, the food has most likely been cooked by professionals who are painstaking in their efforts. In addition, photographers use techniques on food subjects that you might not want to use at home. One example that I remember from years ago was the use of mashed potatoes as a substitute for ice cream--because ice cream would melt too fast under the heat of lights. Food stylists have their hand in these photos, too--it's the equivalent of a person emerging from a high-end spa. Who wouldn't look like a million dollars?!

For icing your cakes, if you are getting crumbs in the frosting, try applying a very thin layer of frosting all over the cake layers to be iced. This catches the crumbs. Then apply the second layer of frosting.