Always Sift Flour
Sifting the flour, even if the bag says the flour was "pre-sifted", makes a world of difference. It incorporates air into the flour and allows the clumped up flour to separate, yielding a smoother finished product. If you don't have a sifter, you could even beat the flour with a fork or whisk.
Warmer eggs are easier to blend than colder ones, and this makes a difference when baking. Simply take out the eggs in advance to allow them to come to room temperature. If you forget to, just run them under warm water for about 30 seconds.
Softened butter creams much more easily that hardened butter, and melted butter doesn't cream at all. You could simply leave the butter out in advance to soften it or place it in or on the oven until it softens.
Beat Until Fluffy
It's really important that you beat the sugar and butter (the usual combination, but it could also include eggs, flour, etc.) until it's actually fluffy. It takes at least 5 minutes, but you'll begin to notice the difference.
Resist The Urge To Peek
When something is baking, just don't do it. Every time you open the oven, you let out some of the heat and reduce the temperature, which affects the baking and baking time. Relax; if it's in the oven, it will bake.
Check Your Oven Temp
Using a thermometer to measure your oven's temperature helps to improve accuracy because ovens are often not what the temperature aims to be. Oven temperature is affected by factors such as elevation and outside temperatures.
Use Quality Ingredients
Using quality ingredients is the only way to ensure quality foods. One of my favorite quotes for this reason is, "No combination of bad eggs has ever made a good omelet." Paying a little extra for real unsalted butter over margarine, for example, is worth every bite.
Follow The Measurements
Baking is a science, so there isn't a lot of room for alterations without changing the final result. When following a recipe, it is best to follow it completely at least once before trying different variations.
Use A Scale
Weight measurements are generally more precise than cup measurements. This improved accuracy and consistency create a final product that is closer to what you expect from the recipe.
Mise En Place
Mise en place is a phrase well-known in the food industry that roughly translates to mean "to put in it's place". In the kitchen, it means to prepare your ingredients and equipment before beginning. If you have all of your ingredients measured before you begin, you're less likely to make mistakes and can proceed from start to finish more smoothly. It also helps to know if you don't actually have everything you need before you start.
Set A Timer
This helps with resisting the urge to peek and keeps you from forgetting about the cake you just spent 30 minutes preparing. While there's no exactly consistent time that any particular dish will take to bake, most recipes provide an estimate. Set a timer to the low end of the estimate and relax.