On ActiveDen we have very high visual standards. Unfortunately, there's no checklist where if you do A, B, and C, you'll have good design. There will be many files that follow the guide below to the letter, but are still rejected for visual reasons. This is just a short list of general rules that you could follow to help improve your design. That said, some of the best designs of all time break all the rules so don't feel as though this list is law.
As the complexity of the file goes down, the visual standards will go up. If your file doesn't do much, it'll need to be gorgeous.
If you'd like comprehensive design tutorials, be sure to check out Psdtuts+. For inspiration, visit Creattica. To see the quality of files that really catch the buyer's eye on ActiveDen, check out the popular files list. If you're ever unsure as to what may or may not be accepted, feel free to ask for advice in the ActiveDen forums.
Simple vs. minimalistic
There's a very fine line between simple and minimalistic. Minimalistic is great! Here's a fantastic example of a minimalistic file.
Files that don't look like anything
Sometimes the type of file you're selling isn't easy to represent visually. This is especially true when it comes to code packages. Let's say you're going to sell a tweening engine. Really the buyer just wants the code, not a final, visual product. That doesn't mean the file preview can't look good. Rather than showing an example of your engine tweening simple shapes, find objects that tween just the same, but look nicer.
Limit the number of fonts
Limit the number of font faces you use within a project. Using too many fonts can make it look messy. Even if you're creating a complete web template, try to use the same two or three font faces in all your pages, tooltips, preloaders, input boxes, and even the sample logo you use.
Make sure that your colors go together. If you're having a hard time coming up with a color scheme, try using a service like Adobe Kuler. Too many colors can make your file look cheap. Unless there's a good reason, your file shouldn't look like a rainbow. Make sure your elements are easily visible and don't blend into the background.
All the way or none at all
If you're trying to make a statement, make sure the viewer will notice it. For example, when placing an isolated object on the stage, either make it centered or definitively off center. If the item is just barely off center, it'll just feel wrong. The same applies to colors and text sizes. Don't make your main text 12pt and your header text 13pt. The most common place I see this is when placing objects next to other objects. It's fine to put your object right next to the something else. It's also fine to give it a decent buffer, just don't give it a two pixel buffer. If you do, it'll probably look funny. Be especially careful when it comes to placing text next to other objects.
Often times assets won't be placed on an exact pixel. For example, an item might be placed at x = 32.2 rather than x = 32. This can make the item look blurry. This is especially true with text. Make sure the items in your file are clear.
Shadows and reflections
If you're going to add a shadow or reflection, make sure they react realistically.
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