Planning how you want you cake to look like and actually drawing your ideas out on paper is an essential step in cake decorating. ALL cute cakes require planning! I almost always draw out a draft of what I want my cake to look like, even if I don't strictly follow it.
Theme: Did your costumer specifically request for a polka-dot cake, or Sesame Street? Are you making your cake for a baby shower? Themes can be very easy to come by (depending on how specific your costumer was) or they may require a more substantial amount of time in your decision making process if the description was vague. Sometimes the theme can be as simple as flowers. Sky's the limit; just make sure you choose a theme and stick to it!
Make sure to balance positive and negative space. Fill up the areas with too much negative space but be sure to keep some negative space; you don't want to distract from the focal point(s). Too many decorations is a common error. Simplicity is best. You've heard this phrase before: Less is more. Couldn't be more true. Always look to see if there is something you can take off your cake, instead of adding. Simplify, simplify, simplify!
Cake decorating is more difficult with regards to balance than say, a painting because cakes as you know are 3-dimensional. It gets especially tricky each tier you add. Generally bigger, more substantial decorations go on the bottom and the opposite is true for the top.
Color also plays an important role in balance (more on that later). Check out this atrocity (thanks Wilton-you know I love to pick on you!). What is the deal with the daisys on the bottom? Their color and the fact they are ALL on the bottom make this cake look extremely awkward. Okay, so maybe its not exactly an atrocity, but for being on the cover of the Wilton course book, you'd think they'd come up with something cuter than this cake. I don't care how perfect their flowers are; I just think its a darn ugly cake.
The problem with many cake decorators (and I've personally witnessed this) is that they tend to slap on a bunch of decorations (flowers, cutouts, whatever) and think the cake will just magically turn out looking right. NOPE. They can have the cutest, most perfect flowers created with the prettiest of colors and impeccable technique, but when they arrange them haphazardly, their cakes look nothing short of an amateurish eyesore. I can't stress balance enough!
Color: Color is one of the first things your viewers will see. This makes your color palate an extremely important element. If your customer has not requested specific colors, the burden will be on your shoulders to decide what looks the best. There are numerous color pallets that are sure to please the eye, likewise, there are many that offend. Most artists choose between complementary colors, warm/cool, neutral, or just one color with it's tints and shades. Of course you don't have to rigidly adhere any of said pallets, they merely serve as a guideline as to how your chosen colors will "feel" and of course, look. Depending on which color theme you choose, not all colors will have and equal voice, so choose carefully.
If any of you have any input, please feel free!