Good T-shirt design needs to be looked at. Eventually you are going to want to get some money from your comic, and you are going to make the realization that, “Hey! I can draw! Time to design a shirt!” As there is nothing that I can do to stop you, I may as well give you some advice.
Keep the design simple. If you get nothing from this, get this: KEEP IT SIMPLE, STUPID. There are some great designs that are very complicated, but the best designs are those that are seen and understood at nearly the same moment. Sure, tour shirts break this rule constantly with that listing on the back, but graphic tees need to make their point almost as soon as you see them. That should be the first test of any design; if it does not make you think or laugh within five seconds of seeing it, it's time to scrap it and go for a different design.
I would point out that there are exceptions to that rule. However, those exceptions combine complicated graphics with a simple point. An Escher print, for example, makes for a great tee shirt design, even though they are all very complicated designs; this is because it conveys a very simply idea that can be grasped immediately and the complicated design emphasizes that point. All I am saying is that a “find Waldo” shirt is going to be a horrible design, unless Waldo is bloody obvious.
The same applies even more so to text. You do not want a viewer to have to stare at the shirt for more than a few moments to read the shirt; this is more of a courtesy to the reader, who should not need to stop to read a shirt, and to the wearer, who should not need to pause so someone can read his shirt. Now, I can see an exception for shirts that you want to draw attention to, but you need to make them the exception, not the rule. It works for some designs, do not get me wrong, but go as light on the text as possible.
Keep in mind also that there are more colors than black and white. White is great because it makes for cheap shirts. Black is great because...yeah, black is just great. But keep in mind that there are other colors out there that people like, such as neon colors. A great option is the baseball tee, which has short colored sleeves and ring shirts, which at least gives you a splash of color. You may not like them, but at least look into other colors and styles than the basic white and black. Please!
[Oh, and while I'm at it: Camisoles. I'm sure that your female potential customers would like something a little sexy and not something that is just a guy's design that has been modified to a camisole. Thank you.]
The last is to keep in mind that the price should be right. Hot Topic can get away with $40 shirts; this is because the people that buy that stuff are on some decent drugs. The price range for most shirts should be in the $10 to $25 range. Combined with simple designs, limited texts, and a few different colors, you should be well on your way to designing a great looking shirt.