Everyone has their own opinion. It really comes down to what your goal is. Are you trying to prove how "out there" and creative you are? Are you trying to sell shirts to make money? Are you trying to advertise a business or event? All of these require specialized techniques to make them the best possible designs they can be. Before that though, here are a few simple rules that can be applied across every category:
- Keep it simple. You don't want passersby to have to spend five minutes studying your shirt. Figure out the message you're trying to convey ("buy this," "isn't this creative," etc.), and figure out how you can communicate that message succinctly.
- Use easy-to-read fonts. A simple enough rule, but one that is oft forgotten.
- Think about your color choices. Make sure the colors within your design complement each other, and that the design as a whole contrasts with the color of your shirt, so that it stands out.
- High resolution. Since you are probably an amateur designer, it is easy to forget to make sure your design is of the highest resolution possible. A design that looks good on the computer may come out pixel-y on your actual t-shirts. A good rule of thumb is to make the graphic at a resolution of at least 300 DPI (dots per inch).
If you are interested in more professional-centered design tips, Loren Sciaky has a great site with really informative professional design tips. Finally, the ooshirts guides provide countless design suggestions for amateur artists. Take a look at all of the resources available, and please share your own design tips in the comments.