Best practices are a piece of cake
While no one step of making a cake is complicated, following best practices each step of the way can help guarantee that the finished product has a moist and tender crumb. Click through this slideshow for tips and tricks for adding one of the most critical ingredients to a cake: eggs.
Warm the eggs
Whenever you mix a cake, it's a good idea to bring all of your ingredients to room temperature. A great way to bring eggs up to temperature quickly is to immerse them in hot (95-116 degree) water for a few minutes. This helps the liquid in the eggs emulsify more easily when introduced to a room temperature fat (creamed butter or oil).
Crack eggs into a transparent container with a spout
For ease of use, I like to add all of my eggs (and whites or yolks too, if they are to be added at the same time) to a container with a pouring spout. This allows me to add the eggs by pouring them slowly into the batter without having to stop and crack them, and ensures that I have the right amount.
Check for shells
I use a transparent container because it allows me to check for stray bits of shell. If there is shell present, it will sink to the bottom of the mixture. I hold the container over my head to inspect it.
Add flavoring to eggs
If the recipe calls for the addition of an extract, add it to the eggs before you begin mixing. It's a great way to evenly infuse the flavor, and a great way to remember to add it at all (I often forget the vanilla, and this tip helps me remember).
Add the eggs one at a time, or as directed
Eggs should be added slowly, to promote emulsion. Most recipes will direct you to add them one or two at a time, but if you encounter a recipe that does not specify, add them one at a time. Beat the batter on the highest speed you can (without splattering) for a minute or two between each addition.
Thoroughly scrape down the bowl
I am an obsessive bowl scraper, and I am especially zealous when it comes to cake batter. The slime of egg whites will cling undetected to the sides and bottom of your mixing bowl, creating streaks in your finished cake. Nip it in the bud by scraping all along the sides and bottom (yes, really) of the bowl between egg additions (or, if adding a lot of eggs, every other addition).
And don't forget the paddle
Don't forget to periodically scrape down the paddle attachment too. This is often where stubborn globs of butter and sugar hang out until the end, creating holes and divots in your cake.
Fixing broken emulsions with heat
Even when you bring your ingredients to room temperature, sometimes the room is too cold to promote emulsion. When yours looks curdled, even after minutes of beating on high speed, soak a towel under the hottest water you can stand to touch, wring it out, and wrap it around the mixing bowl. This will help bring the bowl temperature up and create an emulsion.
Fixing broken emulsions with dry ingredients
Sometimes, emulsions break for no apparent reason; you'll be going along, adding eggs and the batter is looking smooth at an even 70 degrees, and then poof, weepy and broken. When this happens, lower the speed of the mixer, add a few tablespoons of the dry ingredient mixture to the batter, and bring the speed back up gradually, until you're working on high speed again. Continue adding additional tablespoons one at a time as necessary, until the mixture is smooth and homogenous. Scrape down the sides and paddle, give it another high speed mix, and then, proceed as the recipe directs.