I teach design at a top university--just intro drawing courses, mind you. I'm not full of myself. But I do get it, really. I see lots of designs with certain print potential that will never be printed.

A very basic tip is this: know (exactly) what you're doing, and know this by studying others. You shouldn't know this early in the process, but by the time you submit, everything should be working. Even the most casual designs that are good have been toiled over in some way.

Study the shirts you rate 5$--the ones you love, love, love! Why are they working? Is it the idea? The message? Wit? Intellect? Perseverance? Is it the form and execution? Is color important? Many of the designs that don't get printed could, but they sorely lack the impressive point that would earn them enough 5$ to get printed. Use this study, and force yourself to critique and seriously revise (not retouch) your designs so they hit on all cylinders. Whatever isn't working, you need to make that work, and you must, must scrap whatever doesn't work.

Hope this helps.