Make sure thing have good flow. Always have depth, for instance, don't have the whole picture in the very front make some things smaller and give it look like it's farther away. When picking a font, don't get one that has effects already integrated into it, that will just make your image look horrid. Also stick to a color scheme, if you're using a warm color, only use warm colors. Don't try and mix neon orange with dark green, a better combo would be neon orange with neon green. Make sure to stick with a theme, if you are using space stocks or render, USE space renders or stocks don't throw an animal in there for giggles. It will make your image horrid.

Sorry for this mass of words with bad punctuation. I'm too lazy to make it look all nice.